In addition to the myometrium, the amnion synthesizes several bioactive pep tides and prostaglandins that cause myometrial relaxation or contraction (see Fig. 21-1). Late in pregnancy, amnionic prostaglandin biosynthesis is increased, and phospholipase A2 and PGHS-2 show greater activity Oohnson, 2002). Accordingly, many hypothesize that prostaglandins regulate events leading to parturition. The amnion is likely the major source for amnionic fluid prostaglandins, and their role in the activation of cascades that promote membrane rupture is clear. he influence of amnion-derived prostaglandins on uterine quiescence and activation, however, is less delineated. his is because prostaglandin transport from the amnion through the chorion to access maternal tissues is limited by expression of PGDH.
Physiology of Labor 403
Importantly, the phases of parturition should not be confused with the clinical stages of labor, that is, the first, second, and third stages-which make up phase 3 of parturition (Fig. 2i1-4).
arturitionContractile 1unresponsiveness, Uterine of labor cervical softening Phase 3 Stimulation Processes Uterine Phase4 Involution Parturient Uterine preparedness contraction, involution, for labo, cervical dilation, cervical repai, tion, a remarkably efective period of expUlsion (three myometrial quiescence is imposed. This stages of labor)
Initiation of maintenance of cervical structural integ parturition Onset of labor
FIGURE 21-3 The phases of parturition.
PHASE 1: UTERINE QUIESCENCE AND CERVICAL SOFTENING
As shown in Figure 21-3, 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.
Stages of Labor : Phase of of pregnancy and is characterized by
Delivery of uterine smooth muscle tranquility with rity (Fig. 21-5). All manner of molecular systems-neural, endocrine, paracrine, and autocrine-are likely called 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.
During phase 1, the myometrial cells undergo a phenotypic modification to a noncontractile state, and uterine muscle is rendered unresponsive to natural stimuli. Concurrently, the uterus must initiate extensive changes in its size and vascularity to accommodate fetal growth and prepare for uterine :-6
FIGURE 21-4 Composite of the average dilation 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 parts: an acceleration phase, a linear phase of maximum slope, and a deceleration phase. (Redrawm from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, Appleton-Century-Crofts, 1978.) contractions. he myometrial unre sponsiveness of phase 1 continues until near the end of pregnancy. That said, tractions are felt during the quiescent phase, but they do not normally cause cervical dilation. hese contractions are common toward the end of preg nancy, especially in multiparas, and are referred to as Braxton Hicks contractions oralse labor (Chap. 4, p. 50).