Which physiologic event is the key indicator of the commencement of true labor?

Introduction

The last few hours of human pregnancy are characterized by forceful and painful uterine contractions that effect cervical dilatation and cause the fetus to descend through the birth canal. There are extensive preparations in both the uterus and cervix long before this. During the first 36 to 38 weeks of normal gestation, the myometrium is in a preparatory yet unresponsive state. Concurrently, the cervix begins an early stage of remodeling—termed softening—yet maintains structural integrity. Following this prolonged uterine quiescence, there is a transitional phase during which myometrial unresponsiveness is suspended, and the cervix undergoes ripening, effacement, and loss of structural integrity.

The physiological processes that regulate parturition and the onset of labor continue to be defined. It is clear, however, that labor onset represents the culmination of a series of biochemical changes in the uterus and cervix. These result from endocrine and paracrine signals emanating from both mother and fetus. Their relative contributions vary between species, and it is these differences that complicate elucidation of the exact factors that regulate human parturition. When parturition is abnormal, then preterm labor, dystocia, or postterm pregnancy may result. Of these, preterm labor remains the major contributor to neonatal mortality and morbidity in developed countries.

Phases of Parturition

The bringing forth of young—parturition—requires well-orchestrated transformations in both uterine and cervical function. As shown in Figure 21-1, parturition can be arbitrarily divided into four overlapping phases that correspond to the major physiological transitions of the myometrium and cervix during pregnancy (Casey, 1993, 1997; Challis, 2000; Word, 2007). These phases of parturition include: (1) a prelude to it, (2) the preparation for it, (3) the process itself, and (4) recovery. Importantly, the phases of parturition should not be confused with the clinical stages of labor, that is, the first, second, and third stages—which comprise the third phase of parturition (Fig. 21-2).

Figure 21-1

The phases of parturition.

Figure 21-2

Composite of the average dilatation curve for labor in nulliparous women. The curve is based on analysis of data derived from a large, nearly consecutive series of women. The first stage is divided into a relatively flat latent phase and a rapidly progressive active phase. In the active phase, there are three identifiable component parts: an acceleration phase, a linear phase of maximum slope, and a deceleration phase. (Redrawn from Friedman, 1978.)

Phase 1 of Parturition: Uterine Quiescence and Cervical Softening

Uterine Quiescence

Beginning even before implantation, a remarkably effective period of myometrial quiescence is imposed. This phase normally comprises 95 percent of pregnancy and is characterized by uterine smooth muscle tranquility with maintenance of cervical structural integrity. The inherent propensity of the myometrium to contract is held in abeyance, and uterine muscle is rendered unresponsive to natural stimuli. Concurrently, the uterus must initiate extensive changes in its size and vascularity to accommodate the pregnancy and prepare for uterine contractions. The myometrial unresponsiveness of phase 1 continues until near the end of pregnancy. Some low-intensity myometrial contractions are felt during the quiescent phase, but they do not normally cause cervical dilatation. Contractions of this type become more common toward the end of pregnancy, especially in multiparous women, and are referred to as Braxton Hicks contractions or false labor (Chap. 4, Myocyte Arrangement).

Cervical Softening

The cervix has multiple functions during pregnancy that include: (1) maintenance of barrier function to protect the reproductive tract from infection, (2) maintenance of cervical competence despite increasing gravitational forces, and (3) orchestration of extracellular matrix changes that allow progressive increases in tissue compliance.

In nonpregnant women, the cervix is closed and firm, and its consistency is similar to nasal cartilage. By the end of pregnancy, the cervix is easily distensible, and its consistency is similar to the lips of the oral cavity. Thus, the first stage of this remodeling—termed softening—is characterized by an increase in tissue compliance, yet the cervix remains firm and unyielding. Hegar (1895) first described palpable softening of the lower uterine segment at 4 to 6 weeks’ gestation, and this sign was once used to diagnose pregnancy.

Clinically, the maintenance of cervical anatomical and structural integrity is essential for continuation of pregnancy to term. Preterm cervical dilatation, structural incompetence, or both may forecast delivery (Iams, 1996).

Structural Changes with Softening

Cervical softening results from increased vascularity, stromal hypertrophy, glandular hypertrophy and hyperplasia, and slow, progressive compositional or structural changes of the extracellular matrix (House, 2009; Leppert, 1995; Mahendroo, 2012; Word, 2007). During matrix changes, collagen, the main structural protein in the cervix, undergoes conformational changes that alter tissue strength and flexibility. Specifically, collagen processing and the number or type of covalent cross-links between collagen triple helices are altered. These cross-links are normally required for stable collagen fibril formation (Canty, 2005). A reduction in cross-links between newly synthesized collagen monomers results from reduced expression and activity of the cross-link forming enzymes, lysyl hydroxylase and lysyl oxidase, beginning in early pregnancy (Akins, 2011; Drewes, 2007; Ozasa, 1981). Concurrently there is reduced expression of the matricellular proteins thrombospondin 2 and tenascin C. These proteins also influence collagen fibril structure and strength. Together, these early pregnancy changes contribute to the gradual increase in tissue compliance during pregnancy.

The clinical importance of these matrix changes is supported by the greater prevalence of cervical insufficiency in those with inherited defects in collagen and elastin synthesis or assembly (Anum, 2009; Hermanns-Lê, 2005; Paternoster, 1998; Rahman, 2003; Wang, 2006). Examples are Ehlers-Danlos and Marfan syndromes, discussed in Chapter 59 (Hereditary Connective-Tissue Disorders). Additionally, human cervical stromal cells express a transcription factor, microphthalmia-associated transcription factor (MiTF-Cx). During pregnancy, this factor maintains cervical competency by repressing the expression of genes involved in cervical dilation and parturition (Hari Kishore, 2012).

Phase 2 of Parturition: Preparation for Labor

To prepare for labor, the myometrial tranquility of phase 1 of parturition must be suspended—so-called uterine awakening or activation. This phase 2 is a progression of uterine changes during the last 6 to 8 weeks of pregnancy. Importantly, shifting events associated with phase 2 can cause either preterm or delayed labor.

Myometrial Changes

Phase 2 myometrial changes prepare it for labor contractions. This shift probably results from alterations in the expression of key proteins that control contractility. These contraction-associated proteins (CAPs) include the oxytocin receptor, prostaglandin F receptor, and connexin 43 (Smith, 2007). Thus, myometrial oxytocin receptors markedly increase along with increased numbers and surface areas of gap junction proteins such as connexin 43. Together, these lead to increased uterine irritability and responsiveness to uterotonins—agents that stimulate contractions.

Another critical change in phase 2 is formation of the lower uterine segment from the isthmus. With this development, the fetal head often descends to or even through the pelvic inlet—so-called lightening. The abdomen commonly undergoes a shape change, sometimes described by women as “the baby dropped.” It is also likely that the lower segment myometrium is unique from that in the upper uterine segment, resulting in distinct roles for each during labor. This is supported by baboon studies that demonstrate differential expression of prostaglandin receptors within myometrial regions. There are also human studies that report an expression gradient of oxytocin receptors, with greater expression in fundal myometrial cells (Fuchs, 1984; Havelock, 2005; Smith, 2001).

Cervical Ripening During Phase 2

Before contractions begin, the cervix must undergo more extensive remodeling. This eventually results in cervical yielding and dilatation upon initiation of forceful uterine contractions. Cervical modifications during this second phase principally involve connective tissue changes—so-called cervical ripening. The transition from the softening to the ripening phase begins weeks or days before onset of contractions. During this transformation, the total amount and composition of proteoglycans and glycosaminoglycans within the matrix are altered. Many of the processes that aid cervical remodeling are controlled by the same hormones regulating uterine function. That said, the molecular events of each are varied because of differences in cellular composition and physiological requirements. The uterine corpus is predominantly smooth muscle, whereas the cervix is primarily connective tissue. Cellular components of the cervix include fibroblasts, epithelia, and few smooth muscle cells.

Endocervical Epithelia

During pregnancy, endocervical epithelial cells proliferate such that endocervical glands occupy a significant percentage of cervical mass. The endocervical canal is lined with mucus-secreting columnar and stratified squamous epithelia, which protect against microbial invasion. Mucosal epithelia function as sentinels for antigens by expressing Toll-like receptors that recognize pathogens. In addition, epithelia respond in ways that lead to bacterial and viral killing. For this, the epithelia express antimicrobial peptides and protease inhibitors and signal to underlying immune cells when a pathogenic challenge exceeds their protective capacity (Wira, 2005).

In mice, studies suggest that cervical epithelia may also aid cervical remodeling by regulating tissue hydration and maintenance of barrier function. Hydration may be regulated by expression of aquaporins—water channel proteins. Maintenance of barrier function and paracellular transport of ion and solutes is regulated by tight junction proteins, such as claudins 1 and 2 (Anderson, 2006; Timmons, 2007). In the human cervical and vaginal mucosal epithelia, junctional proteins are also reported to be expressed (Blaskewicz, 2011).

Cervical Connective Tissue

The cervix is an extracellular matrix-rich tissue. Constituents of the matrix include type I, III, and IV collagen, glycosaminoglycans, matricellular proteins, proteoglycans, and elastin. Of these, collagen is largely responsible for structural disposition of the cervix. Collagen is the most abundant mammalian protein and has a complex biosynthesis pathway that includes at least six enzymes and chaperones to accomplish maturation. Each collagen molecule is composed of three alpha chains, which wind around each other to form procollagen. Multiple collagen triple-helical molecules are cross-linked to one another by the actions of lysyl oxidase to form fibrils. Collagen fibrils interact with small proteoglycans such as decorin or biglycan, as well as matricellular proteins such as thrombospondin 2. These interactions determine fibril size, packing, and organization (Fig. 21-3). This ensures that collagen fibrils are of uniform diameter and are packed together in a regular and highly organized pattern (Canty, 2005).

Figure 21-3

Fibrillar collagen synthesis and organization. Collagen fibrils are assembled into collagen fibers. Fibril size and packing are regulated in part by small proteoglycans such as decorin that bind collagen. Before cervical ripening, fibril size is uniform, and fibrils are well packed and organized. During cervical ripening, fibril size is less uniform, and spacing between collagen fibrils and fibers is increased and disorganized.

During cervical ripening, collagen fibril diameter is increased, and there is increased spacing between fibrils. These changes may result in part from accumulation of poorly cross-linked collagen and reduced expression of matricellular proteins. Dispersion of collagen fibrils leads to a loss of tissue integrity and increased tissue compliance. Matrix metalloproteases (MMPs) are proteases capable of degrading extracellular matrix proteins. Of these, collagenase members of the MMP family degrade collagen. Some studies support a role of MMPs in cervical ripening. But, others suggest that the biomechanical changes are not consistent solely with collagenase activation and loss of collagen. For example, Buhmschi and colleagues (2004) performed tissue biomechanical studies in the rat and suggest that ripening correlates with changes in the three-dimensional structure of collagen rather than its degradation by collagenases. Moreover, mouse and human studies document no changes in collagen content between nonpregnancy and term pregnancy (Akins, 2011; Myers, 2008; Read, 2007).

Thus, it is likely that dynamic changes in collagen structure rather than collagen content may regulate remodeling. This point is well illustrated in specialized microscopy images of mouse and human cervical collagen (Zhang, 2012). In further support, polymorphisms or mutations in genes required for collagen assembly are associated with an increased incidence of cervical insufficiency (Anum, 2009; Paternoster, 1998; Rahman, 2003; Warren, 2007).

Glycosaminoglycans (GAGs)

These are high-molecular-weight polysaccharides that complex with proteins to form proteoglycans. One glycosaminoglycan is hyaluronan (HA), a carbohydrate polymer whose synthesis is carried out by hyaluronan synthase isoenzymes. Expression of these enzymes is increased in the cervix during ripening (Akgul, 2012; Osmers, 1993; Straach, 2005). The functions of hyaluronans are dependent on size, and the breakdown of large- to small-molecular-weight molecules is carried out by a family of hyaluronidase enzymes. Hyaluronidase genes are expressed in both the mouse and human cervix, and increased hyaluronidase activity is reported in the mouse cervix at term (Akgul, 2012). Large-molecular-weight HA predominates in the mouse cervix during ripening and has a dynamic role to increase viscoelasticity and matrix disorganization. Low-molecular-weight HA has proinflammatory properties, and studies in mice and women reveal increased concentrations during labor and in the puerperium (Akgul, 2012; Ruscheinsky, 2008). The importance of regulated changes in HA size during cervical ripening and dilatation is supported by a study reporting hyaluronidase administration to the cervix for ripening in term pregnant women (Spallicci, 2007). Activation of intracellular signaling cascades and other biological functions requires interactions with cell-associated HA-binding proteins such as versican (Ruscheinsky, 2008).

Proteoglycans

These glycoproteins are composed of a protein core and GAG chains. Changes in the amount of core protein or in the number, length, or degree of sulfation of GAG chains can influence proteoglycan function. Although not well-defined, changes in proteoglycan composition are thought to accompany cervical ripening. At least three small leucine-rich proteoglycans are expressed in the cervix—decorin, biglycan, and fibromodulin (Westergren-Thorsson, 1998). In other connective tissues, decorin and other family members interact with collagen and influence the packing and order of collagen fibrils (Ameye, 2002). Collagen fibrils are rearranged in the skin of decorin-deficient mice and result in collagen fibers that are weakened, shortened, and disorganized (see Fig. 21-3). In addition to the cervix, these proteoglycans are expressed in the fetal membranes and uterus. Changes in expression levels may regulate fetal membrane tensile strength and uterine function (Meiner, 2007; Wu, 2012).

Inflammatory Changes

The marked changes within the extracellular matrix during cervical ripening in phase 2 are accompanied by stromal invasion with inflammatory cells. This has led to a model in which cervical ripening is considered an inflammatory process. As such, cervical chemoattractants attract inflammatory cells, which in turn release proteases that may aid degradation of collagen and other matrix components. In phase 3 or 4 of parturition, there is increased cervical expression of chemokines and collagenase/protease activity. It was assumed that processes regulating phases 3 and 4 of dilation and postpartum recovery of the cervix were similar to those in phase 2 of cervical ripening (Bokström, 1997; Osman, 2003; Sennström, 2000; Young, 2002). This has been challenged by observations from both human and animal studies. Sakamoto and associates (2004, 2005) found no correlation between the degree of clinical cervical ripening and the tissue concentrations of cervical neutrophil-chemoattractant interleukin 8 (IL‑8). Microarray studies comparing gene expression patterns at term before and after cervical ripening report little increase in expression of proinflammatory genes. In contrast, there is a robust increase in proinflammatory and immunosuppressive genes in the cervix after delivery compared with during cervical ripening (Bollapragada, 2009; Hassan, 2006, 2009).

In mouse models, monocyte migration, but not activation, takes place before labor (Timmons, 2006, 2007, 2009). Mice deficient in the chemokine receptor CCR2, important in monocyte homing to tissues, have normally timed labor. This further supports the suggestion that labor is not initiated by an inflammatory response (Menzies, 2012). Furthermore, tissue depletion of neutrophils before birth has no effect on the timing or success of parturition. Finally, activation of neutrophils, proinflammatory M1 macrophages, and alternatively of activated M2 macrophages is increased within 2 hours after birth. This suggests a role for inflammatory cells in postpartum cervical remodeling and repair.

Induction and Prevention of Cervical Ripening

There are no therapies to prevent premature cervical ripening. Cervical cerclage is used to circumvent cervical insufficiency, although success appears limited (Owen, 2012). In contrast, treatment to promote cervical ripening for labor induction includes direct application of prostaglandins E2 (PGE2) and F2α (PGF2α). Prostaglandins likely modify extracellular matrix structure to aid ripening. Although the role of prostaglandins in the normal physiology of cervical ripening remains unclear, this property is useful clinically to assist labor induction (Chap. 26, Pharmacological Techniques). In some nonhuman species, the cascades of events that allow cervical ripening are induced by decreasing serum progesterone concentrations. And in humans, administration of progesterone antagonists causes cervical ripening. As discussed later, humans may have developed unique mechanisms to localize decreases in progesterone action in the cervix and myometrium.

Phase 3 of Parturition: Labor

This phase is synonymous with active labor, which is customarily divided into three stages. These compose the commonly used labor graph shown in Figure 21-2. The clinical stages of labor may be summarized as follows. The first stage begins when spaced uterine contractions of sufficient frequency, intensity, and duration are attained to bring about cervical thinning, or effacement. This labor stage ends when the cervix is fully dilated—about 10 cm—to allow passage of the term-sized fetus. The first stage of labor, therefore, is the stage of cervical effacement and dilatation.

The second stage begins when cervical dilatation is complete and ends with delivery. Thus, the second stage of labor is the stage of fetal expulsion. Last, the third stage begins immediately after delivery of the fetus and ends with the delivery of the placenta. Thus, the third stage of labor is the stage of placental separation and expulsion.

First Stage of Labor: Clinical Onset of Labor

In some women, forceful uterine contractions that effect delivery begin suddenly. In others, labor initiation is heralded by spontaneous release of a small amount of blood-tinged mucus from the vagina. This extrusion of the mucus plug that had previously filled the cervical canal during pregnancy is referred to as “show” or “bloody show.” There is very little blood with the mucous plug, and its passage indicates that labor is already in progress or likely will ensue in hours to days.

Uterine Labor Contractions

Unique among physiological muscular contractions, those of uterine smooth muscle during labor are painful. The cause of this is not known definitely, but several possibilities have been suggested: (1) hypoxia of the contracted myometrium—such as that with angina pectoris; (2) compression of nerve ganglia in the cervix and lower uterus by contracted interlocking muscle bundles; (3) cervical stretching during dilatation; and (4) stretching of the peritoneum overlying the fundus.

Of these, compression of nerve ganglia in the cervix and lower uterine segment by the contracting myometrium is an especially attractive hypothesis. Paracervical infiltration with local anesthetic usually produces appreciable pain relief with contractions (Chap. 25, Paracervical Block). Uterine contractions are involuntary and, for the most part, independent of extrauterine control. Neural blockade from epidural analgesia does not diminish their frequency or intensity. In other examples, myometrial contractions in paraplegic women and in women after bilateral lumbar sympathectomy are normal but painless.

Mechanical stretching of the cervix enhances uterine activity in several species, including humans. This phenomenon has been referred to as the Ferguson reflex (Ferguson, 1941). Its exact mechanism is not clear, and release of oxytocin has been suggested but not proven. Manipulation of the cervix and “stripping” the fetal membranes is associated with an increase in blood levels of prostaglandin F2α metabolite (PGFM).

The interval between contractions diminishes gradually from approximately 10 minutes at the onset of first-stage labor to as little as 1 minute or less in the second stage. Periods of relaxation between contractions, however, are essential for fetal welfare. Unremitting contractions compromise uteroplacental blood flow sufficiently to cause fetal hypoxemia. In active-phase labor, the duration of each contraction ranges from 30 to 90 seconds, averaging about 1 minute. There is appreciable variability in contraction intensity during normal labor. Specifically, amnionic fluid pressures generated by contractions during spontaneous labor average 40 mm Hg, but vary from 20 to 60 mm Hg (Chap. 24, External Monitoring).

Distinct Lower and Upper Uterine Segments

During active labor, the anatomical uterine divisions that were initiated in phase 2 of parturition become increasingly evident (Figs. 21-4 and 21-5). By abdominal palpation, even before membrane rupture, the two segments can sometimes be differentiated. The upper segment is firm during contractions, whereas the lower segment is softer, distended, and more passive. This mechanism is imperative because if the entire myometrium, including the lower uterine segment and cervix, were to contract simultaneously and with equal intensity, the net expulsive force would be markedly decreased. Thus, the upper segment contracts, retracts, and expels the fetus. In response to these contractions, the softened lower uterine segment and cervix dilate and thereby form a greatly expanded, thinned-out tube through which the fetus can pass.

Figure 21-4

Sequence of development of the segments and rings in the uterus at term and in labor. Note comparison between the uterus of a nonpregnant woman, the uterus at term, and the uterus during labor. The passive lower uterine segment is derived from the isthmus, and the physiological retraction ring develops at the junction of the upper and lower uterine segments. The pathological retraction ring develops from the physiological ring. Anat. I.O. = anatomical internal os; E.O. = external os; Hist. I.O. = histological internal os; Ph. R.R. = physiological retraction ring.

Figure 21-5

The uterus at the time of vaginal delivery. The active upper segment retracts around the presenting part as the fetus descends through the birth canal. In the passive lower segment, there is considerably less myometrial tone.

The myometrium of the upper segment does not relax to its original length after contractions. Instead, it becomes relatively fixed at a shorter length. The upper active uterine segment contracts down on its diminishing contents, but myometrial tension remains constant. The net effect is to take up slack, thus maintaining the advantage gained in expulsion of the fetus. Concurrently, the uterine musculature is kept in firm contact with the uterine contents. As the consequence of retraction, each successive contraction commences where its predecessor left off. Thus, the upper part of the uterine cavity becomes slightly smaller with each successive contraction. Because of the successive shortening of the muscular fibers, the upper active segment becomes progressively thickened throughout first- and second-stage labor (see Fig. 21-4). This process continues and results in a tremendously thickened upper uterine segment immediately after delivery.

Clinically, it is important to understand that the phenomenon of upper segment retraction is contingent on a decrease in the volume of its contents. For this to happen, particularly early in labor when the entire uterus is virtually a closed sac with only minimal cervical dilatation, the musculature of the lower segment must stretch. This permits an increasing portion of the uterine contents to occupy the lower segment. The upper segment retracts only to the extent that the lower segment distends and the cervix dilates.

Relaxation of the lower uterine segment mirrors the same gradual progression of retraction. Recall that after each contraction of the upper segment, the muscles do not return to their previous length, but tension remains essentially the same. By comparison, in the lower segment, successive lengthening of the fibers with labor is accompanied by thinning, normally to only a few millimeters in the thinnest part. As a result of the lower segment thinning and concomitant upper segment thickening, a boundary between the two is marked by a ridge on the inner uterine surface—the physiological retraction ring. When the thinning of the lower uterine segment is extreme, as in obstructed labor, the ring is prominent and forms a pathological retraction ring. This abnormal condition is also known as the Bandl ring, which is discussed further and illustrated in Chapter 23 (Complications with Dystocia).

Changes in Uterine Shape During Labor

Each contraction produces an elongation of the ovoid uterine shape with a concomitant decrease in horizontal diameter. This change in shape has important effects on the labor process. First, there is increased fetal axis pressure, that is, the decreased horizontal diameter serves to straighten the fetal vertebral column. This presses the upper pole of the fetus firmly against the fundus, whereas the lower pole is thrust farther downward. The lengthening of the ovoid shape has been estimated at 5 and 10 cm. Second, with lengthening of the uterus, the longitudinal muscle fibers are drawn taut. As a result, the lower segment and cervix are the only parts of the uterus that are flexible, and these are pulled upward and around the lower pole of the fetus.

Ancillary Forces in Labor

After the cervix is dilated fully, the most important force in fetal expulsion is that produced by maternal intraabdominal pressure. Contraction of the abdominal muscles simultaneously with forced respiratory efforts with the glottis closed is referred to as pushing. The force is similar to that with defecation, but the intensity usually is much greater. The importance of intraabdominal pressure is shown by the prolonged descent during labor in paraplegic women and in those with a dense epidural block. And, although increased intraabdominal pressure is necessary to complete second-stage labor, pushing accomplishes little in the first stage. It exhausts the mother, and its associated increased intrauterine pressures may be harmful to the fetus.

Cervical Changes

As the result of contraction forces, two fundamental changes—effacement and dilatation—occur in the already-ripened cervix. For an average-sized fetal head to pass through the cervix, its canal must dilate to a diameter of approximately 10 cm. At this time, the cervix is said to be completely or fully dilated. Although there may be no fetal descent during cervical effacement, most commonly the presenting fetal part descends somewhat as the cervix dilates. During second-stage labor in nulliparas, the presenting part typically descends slowly and steadily. In multiparas, however, particularly those of high parity, descent may be rapid.

Cervical effacement is “obliteration” or “taking up” of the cervix. It is manifest clinically by shortening of the cervical canal from a length of approximately 2 cm to a mere circular orifice with almost paper-thin edges. The muscular fibers at the level of the internal cervical os are pulled upward, or “taken up,” into the lower uterine segment. The condition of the external os remains temporarily unchanged (Fig. 21-6).

Figure 21-6

Schematic showing effacement and dilatation. A. Before labor, the primigravid cervix is long and undilated in contrast to that of the multipara, which has dilatation of the internal and external os. B. As effacement begins, the multiparous cervix shows dilatation and funneling of the internal os. This is less apparent in the primigravid cervix. C. As complete effacement is achieved in the primigravid cervix, dilation is minimal. The reverse is true in the multipara.

Effacement may be compared to a funneling process in which the whole length of a narrow cylinder is converted into a very obtuse, flaring funnel with a small circular opening. Because of increased myometrial activity during uterine preparedness for labor, appreciable effacement of a softened cervix sometimes is accomplished before active labor begins. Effacement causes expulsion of the mucous plug as the cervical canal is shortened.

Because the lower segment and cervix have lesser resistance during a contraction, a centrifugal pull is exerted on the cervix and creates cervical dilatation (Fig. 21-7). As uterine contractions cause pressure on the membranes, the hydrostatic action of the amnionic sac in turn dilates the cervical canal like a wedge. In the absence of intact membranes, the pressure of the presenting fetal part against the cervix and lower uterine segment is similarly effective. Early rupture of the membranes does not retard cervical dilatation so long as the presenting fetal part is positioned to exert pressure against the cervix and lower segment. The process of cervical effacement and dilatation causes formation of the forebag of amnionic fluid. This is the leading portion of fluid and amnionic sac located in front of the presenting part.

Figure 21-7

Hydrostatic action of membranes in effecting cervical effacement and dilatation. With labor progression, note the changing relations of the internal and external os in (A), (B), and (C). Although not shown in this diagram, with membrane rupture, the presenting part, applied to the cervix and the forming lower uterine segment, acts similarly.

Referring back to Figure 21-2, recall that cervical dilatation is divided into latent and active phases. The active phase is subdivided further into the acceleration phase, the phase of maximum slope, and the deceleration phase (Friedman, 1978). The duration of the latent phase is more variable and sensitive to changes by extraneous factors. For example, sedation may prolong the latent phase, and myometrial stimulation shortens it. The latent phase duration has little bearing on the subsequent course of labor, whereas the characteristics of the accelerated phase are usually predictive of labor outcome. Completion of cervical dilatation during the active phase is accomplished by cervical retraction about the presenting part. The first stage ends when cervical dilatation is complete. Once the second stage commences, only progressive descent of the presenting part will foretell further progress.

Second Stage of Labor: Fetal Descent

In many nulliparas, engagement of the head is accomplished before labor begins. That said, the head may not descend further until late in labor. In the descent pattern of normal labor, a typical hyperbolic curve is formed when the station of the fetal head is plotted as a function of labor duration. Station describes descent of the fetal biparietal diameter in relation to a line drawn between maternal ischial spines (Chap. 22, Cervical Assessment). Active descent usually takes place after dilatation has progressed for some time (Fig. 21-8). In nulliparas, increased rates of descent are observed ordinarily during cervical dilatation phase of maximum slope. At this time, the speed of descent is also maximal and is maintained until the presenting part reaches the perineal floor (Friedman, 1978).

Figure 21-8

Labor course divided on the basis of expected evolution of the dilatation and descent curves into three functional divisions. The preparatory division includes the latent and acceleration phases. The dilatational division is the phase of maximum slope of dilatation. The pelvic division encompasses both the deceleration phase and the second stage, which is concurrent with the phase of maximum slope of fetal descent. (Redrawn from Friedman, 1978.)

Pelvic Floor Changes During Labor

The birth canal is supported and is functionally closed by several layers of tissues that together form the pelvic floor. These anatomical structures are shown in detail in Chapter 2 (Superficial Space of the Anterior Triangle). The most important are the levator ani muscle and the fibromuscular connective tissue covering its upper and lower surfaces. There are marked changes in the biomechanical properties of these structures and of the vaginal wall during parturition. These result from altered extracellular matrix structure or composition (Lowder, 2007; Rahn, 2008). The levator ani consists of the pubovisceral, puborectalis, and iliococcygeus muscles, which close the lower end of the pelvic cavity as a diaphragm. Thereby, a concave upper and a convex lower surface are presented. The posterior and lateral portions of the pelvic floor, which are not spanned by the levator ani, are occupied bilaterally by the piriformis and coccygeus muscles.

The levator ani muscle varies in thickness from 3 to 5 mm, although its margins encircling the rectum and vagina are somewhat thicker. During pregnancy, the levator ani usually undergoes hypertrophy, forming a thick band that extends backward from the pubis and encircles the vagina about 2 cm above the plane of the hymen. On contraction, the levator ani draws both the rectum and the vagina forward and upward in the direction of the symphysis pubis and thereby acts to close the vagina.

In the first stage of labor, the membranes, when intact, and the fetal presenting part serve to dilate the upper vagina. The most marked change consists of stretching of the levator ani muscle fibers. This is accompanied by thinning of the central portion of the perineum, which becomes transformed from a wedge-shaped, 5-cm-thick tissue mass to a thin, almost transparent membranous structure less than 1 cm thick. When the perineum is distended maximally, the anus becomes markedly dilated and presents an opening that varies from 2 to 3 cm in diameter and through which the anterior wall of the rectum bulges.

Third Stage of Labor: Delivery of Placenta and Membranes

This stage begins immediately after fetal delivery and involves separation and expulsion of the placenta and membranes. As the neonate is born, the uterus spontaneously contracts around its diminishing contents. Normally, by the time the newborn is completely delivered, the uterine cavity is nearly obliterated. The organ consists of an almost solid mass of muscle, several centimeters thick, above the thinner lower segment. The uterine fundus now lies just below the level of the umbilicus.

This sudden diminution in uterine size is inevitably accompanied by a decrease in the area of the placental implantation site (Fig. 21-9). For the placenta to accommodate itself to this reduced area, it increases in thickness, but because of limited placental elasticity, it is forced to buckle. The resulting tension pulls the weakest layer—decidua spongiosa—from that site. Thus, placental separation follows the disproportion created between the relatively unchanged placental size and the reduced size of the implantation site.

Figure 21-9

Diminution in size of the placental site after birth of the infant. A. Spatial relations before birth. B. Placental spatial relations after birth.

Cleavage of the placenta is aided greatly by the loose structure of the spongy decidua, which may be likened to the row of perforations between postage stamps. As separation proceeds, a hematoma forms between the separating placenta/decidua and the decidua that remains attached to the myometrium. The hematoma is usually the result rather than the cause of the separation, because in some cases bleeding is negligible.

Fetal Membrane Separation and Placental Extrusion

The great decrease in uterine cavity surface area simultaneously throws the fetal membranes—the amniochorion and the parietal decidua—into innumerable folds (Fig. 21-10). Membranes usually remain in situ until placental separation is nearly completed. These are then peeled off the uterine wall, partly by further contraction of the myometrium and partly by traction that is exerted by the separated placenta.

Figure 21-10

Postpartum, membranes are thrown up into folds as the uterine cavity decreases in size. (Photograph contributed by Dr. Kelley S. Carrick.)

After the placenta has separated, it may be expelled by increased abdominal pressure. Completion of the third stage is also accomplished by alternately compressing and elevating the fundus, while exerting minimal traction on the umbilical cord (Fig. 27-12, Anomalous Fetuses). The retroplacental hematoma either follows the placenta or is found within the inverted sac formed by the membranes. In this process, known as the Schultze mechanism of placental expulsion, blood from the placental site pours into the membrane sac and does not escape externally until after extrusion of the placenta. In the other form of placental extrusion, known as the Duncan mechanism, the placenta separates first at the periphery and blood collects between the membranes and the uterine wall and escapes from the vagina. In this circumstance, the placenta descends sideways, and its maternal surface appears first.

Phase 4 of Parturition: The Puerperium

Immediately and for about an hour or so after delivery, the myometrium remains in a state of rigid and persistent contraction and retraction. This directly compresses large uterine vessels and allows thrombosis of their lumens to prevent hemorrhage (Fig. 2-11, Ligaments). This is typically augmented by uterotonics (Chap. 27, Management of the Third Stage).

Uterine involution and cervical repair, both remodeling processes that restore these organs to the nonpregnant state, follow in a timely fashion. These protect the reproductive tract from invasion by commensal microorganisms and restore endometrial responsiveness to normal hormonal cyclicity.

During the early puerperium, there is onset of lactogenesis and milk let-down in mammary glands, as described in Chapter 36 (Breasts and Lactation). Reinstitution of ovulation signals preparation for the next pregnancy. This generally occurs within 4 to 6 weeks after birth, but it is dependent on the duration of breast feeding and lactation-induced, prolactin-mediated anovulation and amenorrhea.

Physiological and Biochemical Processes Regulating Parturition

There are two general contemporaneous theorems concerning labor initiation. Viewed simplistically, the first is the functional loss of pregnancy maintenance factors, whereas the second focuses on synthesis of factors that induce parturition. Some investigators also speculate that the mature fetus is the source of the initial signal for parturition commencement. Others suggest that one or more uterotonins, produced in increased amounts, or an increased population of myometrial uterotonin receptors is the proximate cause. Indeed, an obligatory role for one or more uterotonins is included in most parturition theories, as either a primary or a secondary phenomenon in the final events of childbirth. Both rely on careful regulation of smooth muscle contraction.

Myometrial Action

Anatomical and Physiological Considerations

There are unique characteristics of smooth muscle, including myometrium, compared with those of skeletal muscle that may confer advantages for uterine contraction efficiency and fetal delivery. First, the degree of smooth-muscle cell shortening with contractions may be one order of magnitude greater than that attained in striated muscle cells. Second, forces can be exerted in smooth muscle cells in multiple directions. In contrast, the contraction force generated by skeletal muscle is always aligned with the axis of the muscle fibers. Third, smooth muscle is not organized in the same manner as skeletal muscle. In myometrium, the thick and thin filaments are found in long, random bundles throughout the cells. This plexiform arrangement aids greater shortening and force-generating capacity. Last, greater multidirectional force generation in the uterine fundus compared with that of the lower uterine segment permits versatility in expulsive force directionality. These forces thus can be brought to bear irrespective of the fetal lie or presentation.

Regulation of Myometrial Contraction and Relaxation

Myometrial contraction is controlled by transcription of key genes, which produce proteins that repress or enhance cellular contractility. These proteins function to: (1) enhance the interactions between the actin and myosin proteins that cause muscle contraction, (2) increase excitability of individual myometrial cells, and (3) promote intracellular cross talk that allows development of synchronous contractions.

Actin-Myosin Interactions

The interaction of myosin and actin is essential to muscle contraction. This interaction requires that actin be converted from a globular to a filamentous form. Moreover, actin must be attached to the cytoskeleton at focal points in the cell membrane to allow development of tension (Fig. 21-11). Actin must partner with myosin, which is composed of multiple light and heavy chains. The interaction of myosin and actin activates adenosine triphosphatase (ATPase), hydrolyzes adenosine triphosphate, and generates force. This interaction is brought about by enzymatic phosphorylation of the 20-kDa light chain of myosin (Stull, 1988, 1998). This is catalyzed by the enzyme myosin light-chain kinase, which is activated by calcium. Calcium binds to calmodulin, a calcium-binding regulatory protein, which in turn binds to and activates myosin light-chain kinase.

Figure 21-11

Uterine myocyte relaxation and contraction. A. Uterine relaxation is maintained by factors that increase myocyte cyclic adenosine monophosphate (cAMP). This activates protein kinase A (PKA) to promote phosphodiesterase activity with dephosphorylation of myosin light-chain kinase (MLCK). There are also processes that serve to maintain actin in a globular form, and thus to prevent fibril formation necessary for contractions. B. Uterine contractions result from reversal of these sequences. Actin now assumes a fibrillar form, and calcium enters the cell to combine with calmodulin to form complexes. These complexes activate MLCK to bring about phosphorylation of the myosin light chains. This generates ATPase activity to cause sliding of myosin over the actin fibrils, which is a uterine contractor. AC = adenylyl cyclase; Ca++ = calcium; DAG = diacylglycerol; Gs and Gα = G-receptor proteins; IP3 = inositol triphosphate; LC20 = light chain 20; PIP3 = phosphatidylinositol 3,4,5–triphosphate; PLC = phospholipase C; R-PKA = inactive protein kinase. (Redrawn from Smith, 2007.)

Intracellular Calcium

Agents that promote contraction act on myometrial cells to increase intracellular cytosolic calcium concentration—[Ca2+]i. Or, they allow an influx of extracellular calcium through ligand- or voltage-regulated calcium channels (see Fig. 21-11). For example, prostaglandin F2α and oxytocin bind their respective receptors during labor to open ligand-activated calcium channels. Activation of these receptors also releases calcium from the sarcoplasmic reticulum to cause decreased electronegativity within the cell. Voltage-gated ion channels open, additional calcium ions move into the cell, and cellular depolarization follows. The increase in [Ca2+]i is often transient, but contractions can be prolonged through the inhibition of myosin phosphatase activity (Woodcock, 2004).

Conditions that decrease [Ca2+]i and increase intracellular concentrations of cyclic adenosine monophosphate (cAMP) or cyclic guanosine monophosphate (cGMP) ordinarily promote uterine relaxation. Corticotropin-releasing hormone is one of several factors reported to regulate [Ca2+]i and subsequently modulate expression of the large-conductance potassium channels (BKCa) in the human myometrium (Xu, 2011; You, 2012). Genetic studies in humans and transgenic overexpression in mice reveal that small-conductance calcium-activated K+ isoform 3 (SK3) channels may also be important in maintenance of uterine relaxation (Day, 2011; Rada, 2012). SK3 channel expression declines at the end of term pregnancy as contractility is increased and overexpression of SK3 in transgenic mice dampens uterine contraction force to prevent delivery. Yet another potential mechanism for maintenance of relaxation shown in Figure 21-11 is the promotion of actin in a globular form rather than in fibrils required for contraction (Macphee, 2000; Yu, 1998).

In addition to myocyte contractility, myocyte excitability is also regulated by changes in the electrochemical potential gradient across the plasma membrane. Before labor, myocytes maintain a relatively high interior electronegativity. This state is maintained by the combined actions of the ATPase-driven sodium-potassium pump and the large conductance voltage- and Ca2+-sensitive K channel—maxi-K channel (Parkington, 2001). During uterine quiescence, the maxi-K channel is open and allows potassium to leave the cell to maintain interior electronegativity. At the time of labor, changes in electronegativity lead to depolarization and contraction (Brainard, 2005; Chanrachakul, 2003). And, as parturition progresses, there is increased synchronization of electrical uterine activity.

Myometrial Gap Junctions

Cellular signals that control myometrial contraction and relaxation can be effectively transferred between cells through intercellular junctional channels. Communication is established between myocytes by gap junctions, which aid the passage of electrical or ionic coupling currents as well as metabolite coupling. The transmembrane channels that make up the gap junctions consist of two protein “hemi-channels” (Sáez, 2005). These connexons are each composed of six connexin subunit proteins (Fig. 21-12). These pairs of connexons establish a conduit between coupled cells for the exchange of small molecules that can be nutrients, waste, metabolites, second messengers, or ions.

Figure 21-12

The protein subunits of gap junction channels are called connexins. Six connexins form a hemichannel (connexon), and two connexons (one from each cell) form a gap junction channel. Connexons and gap junction channels can be formed from one or more connexin proteins. The composition of the gap junction channel is important for their selectivity with regard to passage of molecules and communication between cells.

Optimal numbers of gap junctions are believed to be important for electrical myometrial synchrony. Four described in the uterus are connexins 26, 40, 43, and 45. Connexin 43 junctions are scarce in the nonpregnant uterus, and they increase in size and abundance during human parturition (Chow, 1994). Also, mouse models deficient in connexin 43-enriched gap junctions exhibit delayed parturition, further supporting their role (Döring, 2006; Tong, 2009).

Cell Surface Receptors

There are various cell surface receptors that can directly regulate myocyte contractile state. Three major classes are G-protein-linked, ion channel-linked, and enzyme-linked. Multiple examples of each have been identified in human myometrium. These further appear to be modified during the phases of parturition. Most G-protein-coupled receptors are associated with adenylyl cyclase activation. Examples are the CRHR1α and the LH receptors (Fig. 21-13). Other G-protein-coupled myometrial receptors, however, are associated with G-protein-mediated activation of phospholipase C. Ligands for the G-protein-coupled receptors include numerous neuropeptides, hormones, and autacoids. Many of these are available to the myometrium during pregnancy in high concentration via endocrine or autocrine mechanisms (Fig. 21-14).

Figure 21-13

G-protein-coupled receptor signal transduction pathways. A. Receptors coupled to heterotrimeric guanosine-triphosphate (GTP)-binding proteins (G proteins) are integral transmembrane proteins that transduce extracellular signals to the cell interior. G-protein-coupled receptors exhibit a common structural motif consisting of seven membrane-spanning regions. B. Receptor occupation promotes interaction between the receptor and the G protein on the interior surface of the membrane. This induces an exchange of guanosine diphosphate (GDP) for GTP on the G protein α subunit and dissociation of the α subunit from the βγ heterodimer. Depending on its isoform, the GTP-α subunit complex mediates intracellular signaling either indirectly by acting on effector molecules such as adenylyl cyclase (AC) or phospholipase C (PLC), or directly by regulating ion channel or kinase function. cAMP = cyclic adenosine monophosphate; DAG = diacylglycerol; IP3 = inositol triphosphate.

Figure 21-14

Theoretical fail-safe system involving endocrine, paracrine, and autocrine mechanisms for the maintenance of phase 1 of parturition, uterine quiescence. CRH = corticotropin-releasing hormone; hCG = human chorionic gonadotropin; PGE2 = prostaglandin E2; PGI2 = prostaglandin I2; PGDH = 15-hydroxyprostaglandin dehydrogenase.

Cervical Dilatation During Labor

There is a large influx of leukocytes into the cervical stroma with cervical dilatation (Sakamoto, 2004, 2005). Cervical tissue levels of leukocyte chemoattractants such as IL-8 are increased just after delivery, as are IL-8 receptor levels. Identification of genes upregulated just after vaginal delivery further suggests that dilatation and early stages of postpartum repair are aided by inflammatory responses, apoptosis, and activation of proteases that degrade extracellular matrix components (Hassan, 2006; Havelock, 2005). The composition of glycosaminoglycans, proteoglycans, and poorly formed collagen fibrils that were necessary during ripening and dilatation must be rapidly removed to allow reorganization and recovery of cervical structure. In the days that follow parturition, recovery of cervical structure involves processes that resolve inflammation, promote tissue repair, and regenerate dense cervical connective tissue with structural integrity and mechanical strength.

Phase 1: Uterine Quiescence and Cervical Competence

Myometrial quiescence is so remarkable and successful that it probably is induced by multiple independent and cooperative biomolecular processes. Individually, some of these processes may be redundant to ensure pregnancy continuance. It is likely that all manners of molecular systems—neural, endocrine, paracrine, and autocrine—are called on to implement and coordinate a state of relative uterine unresponsiveness. Moreover, a complementary “fail-safe” system that protects the uterus against agents that could perturb the tranquility of phase 1 also must be in place (see Fig. 21-14).

As shown in Figure 21-15, phase 1 of human parturition and its quiescence are likely the result of many factors that include: (1) actions of estrogen and progesterone via intracellular receptors, (2) myometrial cell plasma membrane receptor-mediated increases in cAMP, (3) generation of cGMP, and (4) other systems, including modification of myometrial-cell ion channels.

Figure 21-15

The key factors thought to regulate the phases of parturition. CRH = corticotropin-releasing hormone; hCG = human chorionic gonadotropin; SPA = surfactant protein A. (Adapted from Challis, 2002.)

Progesterone and Estrogen Contributions

In many species, the role of the sex steroid hormones is clear—progesterone inhibits and estrogen promotes the events leading to parturition. In humans, however, it seems most likely that both estrogen and progesterone are components of a broader-based molecular system that implements and maintains uterine quiescence. In many species, the removal of progesterone, that is, progesterone withdrawal, directly precedes progression of phase 1 into phase 2 of parturition. In addition, providing progesterone to some species will delay parturition via a decrease in myometrial activity and continued cervical competency (Challis, 1994). Studies in these species have led to a better understanding of why the progesterone-replete myometrium of phase 1 is relatively noncontractile.

Plasma levels of estrogen and progesterone in normal pregnancy are enormous and in great excess of the affinity constants for their receptors. For this reason, it is difficult to comprehend how relatively subtle changes in the ratio of their concentrations could modulate physiological processes during pregnancy. The teleological evidence, however, for an increased progesterone-to-estrogen ratio in the maintenance of pregnancy and a decline in the progesterone-to-estrogen ratio for parturition is overwhelming. In all species studied to date, including humans, administration of the progesterone-receptor antagonists mifepristone (RU-486) or onapristone will promote some or all key features of parturition. These include cervical ripening, increased cervical distensibility, and increased uterine sensitivity to uterotonins (Bygdeman, 1994; Chwalisz, 1994a; Wolf, 1993).

The exact role of estrogen in regulating human uterine activity and cervical competency is less well understood. That said, it appears that estrogen can act to promote progesterone responsiveness and, in doing so, promote uterine quiescence. The estrogen receptor, acting via the estrogen-response element of the progesterone-receptor gene, induces progesterone-receptor synthesis, which allows increased progesterone-mediated function.

Myometrial Cell-to-Cell Communication

Progesterone maintains uterine quiescence by various mechanisms that cause decreased expression of the contraction-associated proteins (CAPs)(Phase 2 of Parturition: Preparation for Labor). Progesterone can promote expression of the inhibitory transcription factor ZEB1—zinc finger E-box binding homeobox protein 1—which can inhibit expression of the CAP genes, connexin 43, and oxytocin receptor (Renthal, 2010). As another mechanism, progesterone bound to the progesterone receptor (PR) can recruit coregulatory factors. These include PSF—polypyrimidine tract binding protein-associated splicing factor—and Sin3A/HDACs—yeast switch-dependent3 homologue A/histone deacetylase corepressor complex—which inhibit expression of the gene encoding the gap junctional protein connexin 43 in rat and human myocytes (Xie, 2012).

At the end of pregnancy, increased stretch along with increased estrogen dominance results in a decline in PSF and Sin/HDAC levels, thus abrogating the suppression of connexin 43 expression by progesterone. Additionally, with loss of progesterone function at term, ZEB1 levels decline due to increased production of small regulatory RNAs termed microRNAs. This releases inhibition of connexin 43 and oxytocin receptor levels to promote increased uterine contractility (Renthal, 2010; Williams, 2012b).

G-Protein–Coupled Receptors

A number of G-protein-coupled receptors that normally are associated with Gαs-mediated activation of adenylyl cyclase and increased levels of cAMP are present in myometrium. These receptors together with appropriate ligands may act in concert with sex steroid hormones as part of a fail-safe system to maintain uterine quiescence (Price, 2000; Sanborn, 1998).

Beta-Adrenoreceptors

These receptors are prototypical examples of cAMP signaling causing myometrium relaxation. Agents binding to these receptors have been used for tocolysis with preterm labor and include ritodrine and terbutaline (Chap. 42, β-Adrenergic Receptor Agonists). β-Adrenergic receptors mediate Gαs-stimulated increases in adenylyl cyclase, increased levels of cAMP, and myometrial cell relaxation. The rate-limiting factor is likely the number of receptors expressed and the level of adenylyl cyclase expression.

Luteinizing Hormone (LH) and Human Chorionic Gonadotropin (hCG) Receptors

These hormones share the same receptor, and this G-protein-coupled receptor has been demonstrated in myometrial smooth muscle and blood vessels (Lei, 1992; Ziecik, 1992). Levels of myometrial LH-hCG receptors during pregnancy are greater before than during labor (Zuo, 1994). Chorionic gonadotropin acts to activate adenylyl cyclase by way of a plasma membranereceptor–Gαs-linked system. This decreases contraction frequency and force and decreases the number of tissue-specific myometrial cell gap junctions (Ambrus, 1994; Eta, 1994). Thus, high circulating levels of hCG may be one mechanism causing uterine quiescence.

Relaxin

This peptide hormone consists of an A and B chain and is structurally similar to the insulin family of proteins (Bogic, 1995; Weiss, 1995). Relaxin mediates lengthening of the pubic ligament, cervical softening, vaginal relaxation, and inhibition of myometrial contractions. There are two separate human relaxin genes, designated H1 and H2. The H1 gene is primarily expressed in the decidua, trophoblast, and prostate, whereas the H2 gene is primarily expressed in the corpus luteum.

Relaxin in plasma of pregnant women is believed to originate exclusively from corpus luteum secretion. Plasma levels peak at approximately 1 ng/mL between 8 and 12 weeks’ gestation. Thereafter, they decline to lower levels that persist until term. Its plasma membrane receptor—relaxin family peptide receptor 1 (RXFP1)—mediates activation of adenylyl cyclase. Relaxin inhibits contractions of nonpregnant myometrial strips, but not those of uterine tissue taken from pregnant women. It also effects cervical remodeling through cell proliferation and modulation of extracellular matrix components such as collagen and hyaluronan (Park, 2005; Soh, 2012).

Corticotropin-Releasing Hormone (CRH)

This hormone is synthesized in the placenta and hypothalamus. As discussed later, CRH plasma levels increase dramatically during the final 6 to 8 weeks of normal pregnancy and have been implicated in the mechanisms controlling the timing of human parturition (Smith, 2007; Wadhwa, 1998). CRH appears to promote myometrial quiescence during most of pregnancy and aids myometrial contractions with onset of parturition. Recent studies suggest that these opposing actions are achieved by differential actions of CRH via its receptor CRHR1. In the term nonlaboring myometrium, the interaction of CRH with its CRHR1 receptor results in activation of the Gs-adenylate cyclase-cAMP signaling pathway. This results in inhibition of inositol triphosphate (IP3) production and a stabilization of [Ca2+]i (You, 2012). In term laboring myometrium, [Ca2+]i is increased by CRH activation of G proteins Gq and Gi and leads to stimulation of IP3 production and increased contractility. Another aspect of CRH regulation is union of CRH to its binding protein, which can limit bioavailability. CRH-binding protein levels are high during pregnancy and are reported to decline at the time of labor.

Prostaglandins

These interact with a family of eight different G-protein-coupled receptors, several of which are expressed in myometrium (Myatt, 2004). Prostaglandins usually are considered as uterotonins. However, their effects are diverse, and some act as smooth muscle relaxants.

The major synthetic pathways involved in prostaglandin biosynthesis are shown in Figure 21-16. Prostaglandins are produced using plasma membrane-derived arachidonic acid, which usually is released by the action of the phospholipases A2 or C. Arachidonic acid can then act as substrate for both type 1 and type 2 prostaglandin H synthase (PGHS-1 and -2), which are also called cyclooxygenase-1 and -2 (COX-1 and -2). Both PGHS isoforms convert arachidonic acid to the unstable endoperoxide prostaglandin G2 and then to prostaglandin H2. These enzymes are the target of many nonsteroidal antiinflammatory drugs (NSAIDs). Indeed, the tocolytic actions of specific NSAIDs, as discussed in Chapter 42 (β-Adrenergic Receptor Agonists), were considered promising until they were shown to have adverse fetal effects (Loudon, 2003; Olson, 2003, 2007).

Figure 21-16

Overview of the prostaglandin biosynthetic pathway.

Through prostaglandin isomerases, prostaglandin H2 is converted to active prostaglandins, including PGE2, PGF2α, and PGI2. Isomerase expression is tissue-specific and thereby controls the relative production of various prostaglandins. Another important control point for prostaglandin activity is its metabolism, which most often is through the action of 15-hydroxyprostaglandin dehydrogenase (PGDH). Expression of this enzyme can be regulated in the uterus, which is important because of its ability to rapidly inactivate prostaglandins.

The effect of prostaglandins on tissue targets is complicated in that there are a number of G-protein-coupled prostaglandin receptors (Coleman, 1994). This family of receptors is classified according to the binding specificity of a given receptor to a particular prostaglandin. Both PGE2 and PGI2 could potentially act to maintain uterine quiescence by increasing cAMP signaling, yet PGE2 can promote uterine contractility through binding to prostaglandin E receptors 1 and 3 (EP1 and EP3). Also, PGE2, PGD2, and PGI2 have been shown to cause vascular smooth muscle relaxation and vasodilatation in many circumstances. Thus, either the generation of specific prostaglandins or the relative expression of the various prostaglandin receptors may determine myometrial responses to prostaglandins (Lyall, 2002; Olson, 2003, 2007; Smith, 2001; Smith, 1998).

In addition to gestational changes, other studies show that there may be regional changes in the upper and lower uterine segments. COX-2 expression is spatially regulated in the myometrium and cervix in pregnancy and labor, with an increasing concentration gradient from the fundus to the cervix (Havelock, 2005). Thus, it is entirely possible that prostanoids contribute to myometrial relaxation at one stage of pregnancy and to regional fundal myometrial contractions after parturition initiation (Myatt, 2004).

Atrial and Brain Natriuretic Peptides and Cyclic Guanosine Monophosphate (cGMP)

Activation of guanylyl cyclase increases intracellular cGMP levels, which promotes smooth muscle relaxation (Word, 1993). Intracellular cGMP levels are increased in the pregnant myometrium and can be stimulated by atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) receptors, and nitric oxide (Telfer, 2001). All of these factors and their receptors are expressed in the pregnant uterus. However, it remains unclear if these factors and intracellular cGMP play a role in uterine quiescence in normal pregnancy physiology (Itoh, 1994; Yallampalli, 1994a,b).

Accelerated Uterotonin Degradation

In addition to pregnancy-induced compounds that promote myometrial cell refractoriness, there are striking increases in the activity of enzymes that degrade or inactivate endogenously produced uterotonins. Some of these and their degradative enzymes include: PGDH and prostaglandins; enkephalinase and endothelins; oxytocinase and oxytocin; diamine oxidase and histamine; catechol O-methyltransferase and catecholamines; angiotensinases and angiotensin-II; and platelet-activating factor (PAF) acetylhydrolase and PAF. Activities of several of these enzymes are increased by progesterone, and many decrease late in gestation (Bates, 1979; Casey, 1980; Germain, 1994).

Phase 2: Uterine Activation and Cervical Ripening

Classic Progesterone Withdrawal and Parturition

Key factors in uterine activation are depicted in Figure 21-15. In species that exhibit progesterone withdrawal, parturition progression to labor can be blocked by administering progesterone to the mother. However, there are conflicting reports as to whether progesterone administration in pregnant women can delay the timely onset of parturition or prevent preterm labor. The possibility that progesterone-containing injections or vaginal suppositories may be used to prevent preterm labor has been studied in a number of randomized trials conducted during the past 15 years. These are discussed in Chapter 42 (Cervical Cerclage), but in general, their use has marginal clinical benefits in preventing recurrent preterm birth and its associated perinatal morbidity.

Progesterone Receptor Antagonists and Human Parturition

When the steroidal antiprogestin mifepristone (RU-486) is administered during the latter phase of the ovarian cycle, it induces menstruation prematurely. It is also an effective abortifacient during early pregnancy (Chap. 18, Menstrual Aspiration). Mifepristone is a classic steroid antagonist, acting at the level of the progesterone receptor. Although less effective in inducing abortion or labor in women later in pregnancy, mifepristone appears to have some effect on cervical ripening and on increasing myometrial sensitivity to uterotonins (Berkane, 2005; Chwalisz, 1994a,b). These data suggest that humans have a mechanism for progesterone inactivation, whereby the myometrium and cervix become refractory to the inhibitory actions of progesterone.

Functional Progesterone Withdrawal in Human Parturition

As an alternative to classic progesterone withdrawal resulting from decreased secretion, research has focused on mechanisms that inhibit progesterone action in human pregnancy. Functional progesterone withdrawal or antagonism is possibly mediated through several mechanisms: (1) changes in the relative expression of the nuclear progesterone-receptor isoforms, PR-A, PR-B, and PR-C; (2) changes in the relative expression of membrane-bound progesterone receptors; (3) posttranslational modifications of the progesterone receptor; (4) alterations in PR activity through changes in the expression of coactivators or corepressors that directly influence receptor function; (5) local inactivation of progesterone by steroid-metabolizing enzymes or synthesis of a natural antagonist; and (6) microRNA regulation of progesterone-metabolizing enzymes and transcription factors that modulate uterine quiescence.

There is evidence that progesterone-receptor activity is decreased late in gestation. A series of studies have shown that the relative ratio of PR-A to PR-B within the myometrium, decidua, and chorion shifts late in gestation (Madsen, 2004; Mesiano, 2002; Pieber, 2001). Specifically, increased PR-A levels during parturition depress the antiinflammatory actions of PR-B and thereby promote proinflammatory gene expression at term (Tan, 2012). Moreover, these activities have been shown to be specific for the upper and lower uterine segments (Condon, 2003, 2006). Similarly, studies of cervical stroma suggest changes in receptor isoform concentrations (Stjernholm-Vladic, 2004). In addition, membrane PR isoforms are also expressed in the myometrium and placenta. However, it remains to be determined if they play a role to promote the transition from myometrial quiescence to activation (Chapman, 2006; Karteris, 2006; Zachariades, 2012). There is evidence in rodent models that local action of enzymes such as steroid 5α-reductase type 1 or 20α-hydroxysteroid dehydrogenase (20α-HSD) catabolize progesterone to metabolites that have a weak affinity for the progesterone receptor (Mahendroo, 1999; Piekorz, 2005). In the human cervix, decreased activity of 17β-hydroxysteroid dehydrogenase type 2 at term results in a net increase in estrogen and decline in progesterone levels (Andersson, 2008). Recent studies provide new insights into the regulatory role of small noncoding RNAs (microRNAs) in regulating expression of the steroid metabolizing enzyme 20α-HSD (Williams, 2012a). Increased expression of microRNA200a in the term myometrium blunts the expression of STAT5b, an inhibitor of 20α-HSD. Reduced STAT5b function allows increased 20α-HSD levels that result in increased progesterone metabolism and reduced progesterone function.

Taken together, all of these observations support the concept that multiple pathways exist for a functional progesterone withdrawal that includes changes in PR isoform and receptor coactivator levels, microRNA regulation, and increased local hormone metabolism to less active products.

Oxytocin Receptors

Because of its longstanding application for labor induction, it seemed logical that oxytocin must play a central role in spontaneous human labor. But this venerable hormone may have only a minor supporting role. Currently, it still is controversial whether oxytocin plays a role in the early phases of uterine activation or whether its sole function is in the expulsive phase of labor. Most studies of regulation of myometrial oxytocin-receptor synthesis have been performed in rodents. Disruption of the oxytocin receptor gene in the mouse does not affect parturition. This suggests that, at least in this species, multiple systems likely ensure that parturition occurs. There is little doubt, however, that there is an increase in myometrial oxytocin receptors during phase 2 of parturition. Moreover, their activation results in increased phospholipase C activity and subsequent increases in cytosolic calcium levels and uterine contractility.

Progesterone and estradiol appear to be the primary regulators of oxytocin receptor expression. Estradiol treatment in vivo or in myometrial explants increases myometrial oxytocin receptor concentrations. This action, however, is prevented by simultaneous treatment with progesterone (Fuchs, 1983). Progesterone also may act within the myometrial cell to increase oxytocin-receptor degradation and inhibit oxytocin activation of its receptor at the cell surface (Bogacki, 2002). These data indicate that one of the mechanisms whereby progesterone maintains uterine quiescence is through the inhibition of myometrial oxytocin response.

The increase in oxytocin receptor levels in nonhuman species appears to be mainly regulated either directly or indirectly by estradiol. Treatment of several species with estrogen leads to increased uterine oxytocin receptor levels (Blanks, 2003; Challis, 1994). Moreover, the level of oxytocin receptor mRNA in human myometrium at term is greater than that found in preterm myometrium (Wathes, 1999). Thus, increased receptors at term may be attributable to increased gene transcription. An estrogen response element, however, is not present in the oxytocin receptor gene, suggesting that the stimulatory effects of estrogen may be indirect.

Human studies suggest that inflammatory-related rapid-response genes may regulate oxytocin receptors (Bethin, 2003; Kimura, 1999; Massrieh, 2006). These receptors also are present in human endometrium and in decidua at term and stimulate prostaglandin production. In addition, these receptors are found in the myometrium and at lower levels in amniochorion-decidual tissues (Benedetto, 1990; Wathes, 1999).

Relaxin

Although relaxin may contribute to uterine quiescence, it also has roles in phase 2 of parturition. These include remodeling of the extracellular matrix of the uterus, cervix, vagina, breast, and pubic symphysis as well as promoting cell proliferation and inhibiting apoptosis. Its actions on cell proliferation and apoptosis are mediated through the G-protein-coupled receptor, RXFP1, whereas some but not all actions of relaxin on matrix remodeling are mediated through this receptor (Samuel, 2009; Yao, 2008). The precise mechanisms for modulation of matrix turnover have not been fully elucidated. However, relaxin appears to mediate glycosaminoglycan and proteoglycan synthesis and degrade matrix macromolecules such as collagen by induction of matrix metalloproteases. Relaxin promotes growth of the cervix, vagina, and pubic symphysis and is necessary for breast remodeling for lactation. Consistent with its proposed roles, mice deficient in relaxin or the RXFP1 receptor have protracted labor; show reduced growth of the cervix, vagina, and symphysis; and are unable to nurse because of incomplete nipple development (Feng, 2005; Park, 2005; Rosa, 2012; Soh, 2012; Yao, 2008).

Fetal Contributions to Initiation of Parturition

It is intellectually intriguing to envision that the mature human fetus provides the signal to initiate parturition. Teleologically, this seems most logical because such a signal could be transmitted in several ways to suspend uterine quiescence. The fetus may provide a signal through a blood-borne agent that acts on the placenta. Research is ongoing to better understand the fetal signals that contribute to parturition initiation (Mendelson, 2009). Although signals may arise from the fetus, the uterus and cervix likely first must be prepared for labor before a uterotonin produced by or one whose release is stimulated by the fetus can be optimally effective (Casey, 1994).

Uterine Stretch and Parturition

There is now considerable evidence that fetal growth is an important component in uterine activation in phase 1 of parturition. In association with fetal growth, significant increases in myometrial tensile stress and amnionic fluid pressure follow (Fisk, 1992). With uterine activation, stretch is required for induction of specific contraction-associated proteins (CAPs). Specifically, stretch increases expression of the gap junction protein—connexin 43 and of oxytocin receptors. Gastrin-releasing peptide, a stimulatory agonist for smooth muscle, is increased by stretch in the myometrium (Tattersall, 2012). Others have hypothesized that stretch plays an integrated role with fetal-maternal endocrine cascades of uterine activation (Lyall, 2002; Ou, 1997, 1998).

Clinical support for a role of stretch comes from the observation that multifetal pregnancies are at a much greater risk for preterm labor than singletons. And preterm labor is also significantly more common in pregnancies complicated by hydramnios. Although the mechanisms causing preterm birth in these two examples are debated, a role for uterine stretch must be considered.

Cell signaling systems used by stretch to regulate the myometrial cell continue to be defined. This process—mechanotransduction—may include activation of cell-surface receptors or ion channels, transmission of signals through extracellular matrix, or release of autocrine molecules that act directly on myometrium (Shynlova, 2009; Young, 2011). For example, the extracellular matrix protein fibronectin and its cell-surface receptor, alpha 5 integrin receptor, are induced in the rodent by stretch (Shynlova, 2007). This interaction may aid force transduction during labor contraction by anchoring hypertrophied myocytes to the uterine extracellular matrix.

Fetal Endocrine Cascades Leading to Parturition

The ability of the fetus to provide endocrine signals that initiate parturition has been demonstrated in several species. Liggins and associates (1967, 1973) demonstrated that the fetus provides the signal for the timely onset of parturition in sheep. This signal was shown to come from the fetal hypothalamic-pituitary-adrenal axis (Whittle, 2001).

Defining the exact mechanisms regulating human parturition has proven more difficult, and all evidence suggests that it is not regulated in the exact manner seen in the sheep. Even so, activation of the human fetal hypothalamic-pituitary-adrenal-placental axis is considered a critical component of normal parturition. Moreover, premature activation of this axis is considered to prompt many cases of preterm labor (Challis, 2000, 2001). As in the sheep, steroid products of the human fetal adrenal gland are believed to have effects on the placenta and membranes that eventually transform myometrium from a quiescent to contractile state. A key component in the human may be the unique ability of the placenta to produce large amounts of CRH, as shown in Figure 21-17.

Figure 21-17

The placental–fetal adrenal endocrine cascade. In late gestation, placental corticotropin-releasing hormone (CRH) stimulates fetal adrenal production of dehydroepiandrosterone sulfate (DHEA-S) and cortisol. The latter stimulates production of placental CRH, which leads to a feed-forward cascade that enhances adrenal steroid hormone production. ACTH = adrenocorticotropic hormone.

Placental Corticotropin-Releasing Hormone Production

A CRH hormone identical to maternal and fetal hypothalamic CRH is synthesized by the placenta in relatively large amounts (Grino, 1987; Saijonmaa, 1988). One important difference is that, unlike hypothalamic CRH, which is under glucocorticoid negative feedback, cortisol has been shown to stimulate placental CRH production. This is by activation of the transcription factor, nuclear factor kappa B (NF-κB) (Jones, 1989; Marinoni, 1998; Thomson, 2013). This ability makes it possible to create a feed-forward endocrine cascade that does not end until delivery.

Maternal plasma CRH levels are low in the first trimester and rise from midgestation to term. In the last 12 weeks, CRH plasma levels rise exponentially, peaking during labor and then falling precipitously after delivery (Frim, 1988; Sasaki, 1987). Amnionic fluid CRH levels similarly increase in late gestation. CRH is the only trophic hormone-releasing factor to have a specific serum binding protein. During most of pregnancy, it appears that CRH-binding protein (CRH-BP) binds most maternal circulating CRH, and this inactivates it (Lowry, 1993). During later pregnancy, however, CRH-BP levels in both maternal plasma and amnionic fluid decline, leading to markedly increased levels of bioavailable CRH (Perkins, 1995; Petraglia, 1997).

In pregnancies in which the fetus can be considered to be “stressed” from various complications, concentrations of CRH in fetal plasma, amnionic fluid, and maternal plasma are increased compared with those seen in normal gestation (Berkowitz, 1996; Goland, 1993; McGrath, 2002; Perkins, 1995). The placenta is likely the source for this increased CRH concentration (Torricelli, 2011). For example, placental CRH content was fourfold higher in placentas from women with preeclampsia than in those from normal pregnancies (Perkins, 1995). Such increases in placental CRH production during normal gestation and the excessive secretion of placental CRH in complicated pregnancies may play a role in fetal adrenal cortisol synthesis (Murphy, 1982). They also may result in the supranormal levels of umbilical cord blood cortisol noted in stressed neonates (Falkenberg, 1999; Goland, 1994).

Corticotropin-Releasing Hormones and Parturition Timing

Placental CRH has been proposed to play several roles in parturition regulation. Placental CRH may enhance fetal cortisol production to provide positive feedback so that the placenta produces more CRH. Late in pregnancy—phase 2 or 3 of parturition—modification in the CRH receptor favors a switch from cAMP formation to increased myometrial cell calcium levels via protein kinase C activation (You, 2012). Oxytocin acts to attenuate CRH-stimulated accumulation of cAMP in myometrial tissue. And, CRH augments the contraction-inducing potency of a given dose of oxytocin in human myometrial strips (Quartero, 1991, 1992). CRH acts to increase myometrial contractile force in response to PGF2α (Benedetto, 1994). Finally, CRH has been shown to stimulate fetal adrenal C19-steroid synthesis, thereby increasing substrate for placental aromatization. Increased production of estrogens would shift the estrogen-to-progesterone ratio and promote the expression of a series of myometrial contractile proteins.

Some have proposed that the rising level of CRH at the end of gestation reflects a fetal-placental clock (McLean, 1995). CRH levels vary greatly among women, and it appears that the rate of increase in maternal CRH levels is a more accurate predictor of pregnancy outcome than is a single measurement (Leung, 2001; McGrath, 2002). In this regard, the placenta and fetus, through endocrinological events, influence the timing of parturition at the end of normal gestation.

Fetal Lung Surfactant and Parturition

Surfactant protein A (SP-A) produced by the fetal lung is required for lung maturation. Its levels are increased in amnionic fluid at term in mice. Studies in the mouse suggest that the increasing SP-A concentrations in amnionic fluid activate fluid macrophages to migrate into the myometrium and induce NF-κB (Condon, 2004). This factor activates inflammatory response genes in the myometrium, which in turn promote uterine contractility. This model supports the supposition that fetal signals play a role in parturition initiation. SP-A is expressed by the human amnion and decidua, is present in the amnionic fluid, and prompts signaling pathways in human myometrial cells (Garcia-Verdugo, 2008; Lee, 2010; Snegovskikh, 2011). The exact mechanisms by which SP-A activates myometrial contractility in women, however, remains to be clarified (Leong, 2008). SP-A selectively inhibits prostaglandin F2α in the term decidua, and amnionic fluid concentration of SP-A declines at term (Chaiworapongsa, 2008).

Fetal Anomalies and Delayed Parturition

There is fragmentary evidence that pregnancies with markedly diminished estrogen production may be associated with prolonged gestation. These “natural experiments” include fetal anencephaly with adrenal hypoplasia and those with inherited placental sulfatase deficiency. The broad range of gestational length seen with these disorders calls into question the exact role of estrogen in human parturition initiation.

Other fetal abnormalities that prevent or severely reduce the entry of fetal urine into amnionic fluid—renal agenesis, or into lung secretions—pulmonary hypoplasia, do not prolong human pregnancy. Thus, a fetal signal through the paracrine arm of the fetal-maternal communication system does not appear to be mandated for parturition initiation.

Some brain anomalies of the fetal calf, fetal lamb, and sometimes the human fetus delay the normal timing of parturition. More than a century ago, Rea (1898) observed an association between fetal anencephaly and prolonged human gestation. Malpas (1933) extended these observations and described a pregnancy with an anencephalic fetus that was prolonged to 374 days—53 weeks. He concluded that the association between anencephaly and prolonged gestation was attributable to anomalous fetal brain-pituitary-adrenal function. The adrenal glands of the anencephalic fetus are very small and, at term, may be only 5 to 10 percent as large as those of a normal fetus. This is caused by developmental failure of the fetal zone that normally accounts for most of fetal adrenal mass and production of C19-steroid hormones (Chap. 5, Directional Secretion of Steroids from Syncytiotrophoblast). Such pregnancies are associated with delayed labor and suggest that the fetal adrenal glands are important for the timely onset of parturition (Anderson, 1973).

Phase 3: Uterine Stimulation

This parturition phase is synonymous with uterine contractions that bring about progressive cervical dilatation and delivery. Current data favor the uterotonin theory of labor initiation. Increased uterotonin production would follow once phase 1 is suspended and uterine phase 2 processes are implemented. A number of uterotonins may be important to the success of phase 3, that is, active labor (see Fig. 21-15). Just as multiple processes likely contribute to myometrial unresponsiveness of phase 1 of parturition, other processes may contribute jointly to a system that ensures labor success.

Uterotonins that are candidates for labor induction include oxytocin, prostaglandins, serotonin, histamine, PAF, angiotensin II, and many others. All have been shown to stimulate smooth muscle contraction through G-protein coupling.

Oxytocin and Phase 3 of Parturition

Late in pregnancy, during phase 2 of parturition, there is a 50-fold or more increase in the number of myometrial oxytocin receptors (Fuchs, 1982; Kimura, 1996). This increase coincides with an increase in uterine contractile responsiveness to oxytocin. Moreover, prolonged gestation is associated with a delay in the increase of these receptors (Fuchs, 1984).

Oxytocin—literally, quick birth—was the first uterotonin to be implicated in parturition initiation. This nanopeptide is synthesized in the magnocellular neurons of the supraoptic and paraventricular neurons. The prohormone is transported with its carrier protein, neurophysin, along the axons to the neural lobe of the posterior pituitary gland in membrane-bound vesicles for storage and later release. The prohormone is converted enzymatically to oxytocin during transport (Gainer, 1988; Leake, 1990).

Role of Oxytocin in Phases 3 and 4 of Parturition

Because of successful labor induction with oxytocin, it was logically suspected in parturition initiation. First, in addition to its effectiveness in inducing labor at term, oxytocin is a potent uterotonin and occurs naturally in humans. Subsequent observations provide additional support for this theory: (1) the number of oxytocin receptors strikingly increases in myometrial and decidual tissues near the end of gestation; (2) oxytocin acts on decidual tissue to promote prostaglandin release; and (3) oxytocin is synthesized directly in decidual and extraembryonic fetal tissues and in the placenta (Chibbar, 1993; Zingg, 1995).

Although little evidence suggests a role for oxytocin in phase 2 of parturition, abundant data support its important role during second-stage labor and in the puerperium—phase 4 of parturition. Specifically, there are increased maternal serum oxytocin levels: (1) during second-stage labor, which is the end of phase 3 of parturition; (2) in the early puerperium; and (3) during breast feeding (Nissen, 1995). Immediately after delivery of the fetus, placenta, and membranes, which completes parturition phase 3, firm and persistent uterine contractions and myometrial retraction are essential to prevent postpartum hemorrhage. Oxytocin likely causes persistent contractions.

Oxytocin infusion in women promotes increased levels of mRNAs from myometrial genes that encode proteins essential for uterine involution. These include interstitial collagenase, monocyte chemoattractant protein-1, interleukin-8, and urokinase plasminogen activator receptor. Therefore, oxytocin action at the end of labor may be involved in uterine involution.

Prostaglandins and Phase 3 of Parturition

Although their role in parturition phase 2 of noncomplicated pregnancies is less well defined, a critical role for prostaglandins in phase 3 of parturition is clear (MacDonald, 1993). First, levels of prostaglandins—or their metabolites—in amnionic fluid, maternal plasma, and maternal urine are increased during labor (Fig. 21-18. Second, treatment of pregnant women with prostaglandins, by any of several administration routes, causes abortion or labor at all gestational stages. Moreover, administration of prostaglandin H synthase type 2 (PGHS-2) inhibitors to pregnant women will delay spontaneous labor onset and sometimes arrest preterm labor (Loudon, 2003). Last, prostaglandin treatment of myometrial tissue in vitro sometimes causes contraction, dependent on the prostanoid tested and the physiological status of the tissue treated.

Figure 21-18

Mean (±SD) concentrations of prostaglandin F2α (PGF2α) and prostaglandin E2 (PGE2) in amnionic fluid at term before labor and in the upper and forebag compartments during labor at all stages of cervical dilatation. (Data from MacDonald, 1993.)

Uterine Events Regulating Prostaglandin Production

During labor, prostaglandin production within the myometrium and decidua is an efficient mechanism of activating contractions. For example, prostaglandin synthesis is high and unchanging in the decidua during phase 2 and 3 of parturition. Moreover, the receptor level for PGF2α is increased in the decidua at term, and this increase most likely is the regulatory step in prostaglandin action in the uterus. The myometrium synthesizes PGHS-2 with labor onset, but most prostaglandins likely come from the decidua.

The fetal membranes and placenta also produce prostaglandins. Primarily PGE2, but also PGF2α, are detected in amnionic fluid at all gestational stages. As the fetus grows, prostaglandins levels in the amnionic fluid increase gradually. Their major increases in concentration within amnionic fluid, however, are demonstrable after labor begins (see Fig. 21-18). These higher levels likely result as the cervix dilates and exposes decidual tissue (Fig. 21-19). These increased levels in the forebag compared with those in the upper compartment are believed to follow an inflammatory response that signals the events leading to active labor. Together, the increases in cytokines and prostaglandins further degrade the extracellular matrix, thus weakening fetal membranes.

Figure 21-19

Sagittal view of the exposed forebag and attached decidual fragments after cervical dilatation during labor. (Redrawn from MacDonald, 1996.)

Findings of Kemp and coworkers (2002) and Kelly (2002) support a possibility that inflammatory mediators aid cervical dilatation and alterations to the lower uterine segment. It can be envisioned that they, along with the increased prostaglandin levels measured in vaginal fluid during labor, add to the relatively rapid cervical changes that are characteristic of parturition.

Endothelin-1

The endothelins are a family of 21-amino acid peptides that powerfully induce myometrial contraction (Word, 1990). The endothelin A receptor is preferentially expressed in smooth muscle and effects an increase in intracellular calcium. Endothelin-1 is produced in myometrium of term gestations and is able to induce synthesis of other contractile mediators such as prostaglandins and inflammatory mediators (Momohara, 2004; Sutcliffe, 2009). The requirement of endothelin-1 in normal parturition physiology remains to be established. However, there is evidence of pathologies associated with aberrant endothelin-1 expression, such as premature birth and uterine leiomyomas (Tanfin, 2011, 2012).

Angiotensin II

There are two G-protein-linked angiotensin II receptors expressed in the uterus—AT1 and AT2. In nonpregnant women, the AT2 receptor is predominant, but the AT1 receptor is preferentially expressed in pregnant women (Cox, 1993). Angiotensin II binding to the plasma-membrane receptor evokes contraction. During pregnancy, the vascular smooth muscle that expresses the AT2 receptor is refractory to the pressor effects of infused angiotensin II (Chap. 4, Supine Hypotension). In myometrium near term, however, angiotensin II may be another component of the uterotonin system of parturition phase 3 (Anton, 2009).

Contribution of Intrauterine Tissues to Parturition

Although they have a potential role in parturition initiation, the amnion, chorion laeve, and decidua parietalis more likely have an alternative role. The membranes and decidua make up an important tissue shell around the fetus that serves as a physical, immunological, and metabolic shield to protect against untimely initiation of parturition. Late in gestation, however, the fetal membranes may indeed act to prepare for labor.

Amnion

Virtually all of the tensile strength—resistance to tearing and rupture—of the fetal membranes is provided by the amnion (Chap. 5, Immunogenicity of the Trophoblasts). This avascular tissue is highly resistant to penetration by leukocytes, microorganisms, and neoplastic cells. It also constitutes a selective filter to prevent fetal particulate-bound lung and skin secretions from reaching the maternal compartment. In this manner, maternal tissues are protected from amnionic fluid constituents that could worsen decidual or myometrial function or could promote adverse events such as amnionic-fluid embolism (Chap. 41, Amnionic-Fluid Embolism).

Several bioactive peptides and prostaglandins that cause myometrial relaxation or contraction are synthesized in amnion (Fig. 21-20). Late in pregnancy, amnionic prostaglandin biosynthesis is increased, and phospholipase A2 and PGHS-2 show increased activity (Johnson, 2002). Accordingly, many hypothesize that prostaglandins regulate events leading to parturition. It is likely that amnion is the major source for amnionic fluid prostaglandins, and their role in activation of cascades that promote membrane rupture is clear. The influence of amnion-derived prostaglandins on uterine quiescence and activation, however, is less clear. This is because prostaglandin transport from the amnion through the chorion to access maternal tissues is limited by expression of the inactivating enzyme, prostaglandin dehydrogenase.

Figure 21-20

The amnion synthesizes prostaglandins, and late in pregnancy, synthesis is increased by increased phospholipase A2 and prostaglandin H synthase, type 2 (PGHS-2) activity. During pregnancy, the transport of prostaglandins from the amnion to maternal tissues is limited by expression of the inactivating enzymes, prostaglandin dehydrogenase (PGDH), in the chorion. During labor, PGDH levels decline, and amnion-derived prostaglandins can influence membrane rupture and uterine contractility. The role of decidual activation in parturition is unclear but may involve local progesterone metabolism and increased prostaglandin receptor concentrations, thus enhancing uterine prostaglandin actions and cytokine production. (Adapted from Smith, 2007.)

Chorion Laeve

This tissue layer also is primarily protective and provides immunological acceptance. The chorion laeve is also enriched with enzymes that inactivate uterotonins. Enzymes include prostaglandin dehydrogenase (PGDH), oxytocinase, and enkephalinase (Cheung, 1990; Germain, 1994). As noted, PGDH inactivates amnion-derived prostaglandins. With chorionic rupture, this barrier would be lost, and prostaglandins could readily influence adjacent decidua and myometrium.

There is also evidence that PGDH levels found in the chorion decline during labor. This would allow increased prostaglandin-stimulated matrix metalloproteinase activity associated with membrane rupture. It would further allow prostaglandin entry into the maternal compartment to promote myometrial contractility (Patel, 1999; Van Meir, 1996; Wu, 2000). It is likely that progesterone maintains chorion PGDH expression, whereas cortisol decreases its expression. Thus, PGDH levels would decrease late in gestation as fetal cortisol production increases and as part of progesterone withdrawal.

Decidua

A metabolic contribution of decidual activation to parturition initiation is an appealing possibility for both anatomical and functional reasons. The generation of decidual uterotonins that act in a paracrine manner on contiguous myometrium is intuitive. In addition, decidua expresses steroid metabolizing enzymes such as 20α-HSD and steroid 5αR1 that may regulate local progesterone withdrawal. Decidual activation is characterized by increased proinflammatory cells and increased expression of proinflammatory cytokines, prostaglandins, and uterotonins such as oxytocin receptors and connexin 43.

Cytokines produced in the decidua can either increase uterotonin production—principally prostaglandins. Or they can act directly on myometrium to cause contraction. Examples are tumor necrosis factor-α (TNF-α) and interleukins 1, 6, 8, and 12. These molecules also can act as chemokines that recruit to the myometrium neutrophils and eosinophils, which further increase contractions and labor (Keelan, 2003).

There is uncertainty whether prostaglandin concentration or output from the decidua increases with term labor onset. Olson and Ammann (2007) suggest that the major regulation of decidual prostaglandin action is not their synthesis but rather increased PGF2α receptor expression.

Summary: Regulation of Phase 3 and 4 of Parturition

It is likely that multiple and possibly redundant processes contribute to the success of the three active labor phases once phase 1 of parturition is suspended and phase 2 is initiated. Phase 3 is highlighted by increased activation of G-protein-coupled receptors that inhibit cAMP formation, increase intracellular calcium stores, and promote interaction of actin and myosin and subsequent force generation. Simultaneously, cervical proteoglycan composition and collagen structure are altered to a form that promotes tissue distensibility and increased compliance. The net result is initiation of coordinated myometrial contractions of sufficient amplitude and frequency to dilate the prepared cervix and push the fetus through the birth canal. Multiple regulatory ligands orchestrate these processes and vary from endocrine hormones such as oxytocin to locally produced prostaglandins.

In phase 4 of parturition, a complicated series of repair processes are initiated to resolve inflammatory responses and remove glycosaminoglycans, proteoglycans, and structurally compromised collagen. Simultaneously, matrix and cellular components required for complete uterine involution are synthesized, and the dense connective tissue and structural integrity of the cervix is reformed.

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