Cervical facement is "obliteration" or "taking up" of the cervix. It is manifest clinically by shortening of the cervical canal from a length of approximately 3 cm to a mere circular oriice 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. 2i1-13).
FIGURE 21-13 Schematic showing effacement and dilation.
A. Before labor, the primigravid cervix is long and undilated in contrast to that of the mUltipara, which has dilation of the internal and external os. B. As efacement begins, the multiparous cervix shows dilation and funneling of the internal os. This is less apparent in the primigravid cervix. C. As complete efacement is achieved in the primigravid cervix, dilation is minimal. The reverse is true in the multipara.
Efacement 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 distal circular opening. Because of growing myometrial activity during uterine preparedness for labor, appreciable efacement of a softened cervix sometimes is accomplished before active labor begins. Efacement causes expulsion of the mucous plug as the cervical canal is shortened.
Because the lower segment and cervix have less resistance during a contraction, a centrifugal pull is exerted on the cervix and creates cervical dilation (Fig. 21-14). As uterine contractions cause pressure on the membranes, the hydrostatic action of the amnionic sac in turn dilates the cervical canal like a wedge. The process of cervical efacement and dilation causes formation of the orebag of amnionic luid. This is the leading portion of fluid and amnionic sac located in front of the presenting part. In the absence of intact membranes, the pressure of the presenting fetal part against the cervix and lower uterine segment is similarly efective. Early rupture of the membranes does not retard cervical dilation so long as the presenting fetal part is positioned to exert pressure against the cervix and lower segment.
Referring back to Figure 21-4, recall that cervical dilation 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 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. The first stage ends when cervical dilation is complete.
• Second Stage: Fetal Descent
In many nulliparas, engagement of the head is accomplished before labor begins. hat 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, p. 436). Active descent usually takes place after dilation has progressed for some time (Fig. 21-15). During second-stage labor, the speed of descent is maximal and is maintained until the presenting part reaches the perineal floor (Friedman, 1978). In nulliparas, the presenting part typically descends slowly and steadily. In multiparas, however, particularly those of high parity, descent may be rapid.
he birth canal is supported and functionally closed by the pelvic loor (Chap. 2, p. 21). The most important component of the loor is the levator ani muscle and the ibromuscular connective tissue that covers its upper and lower surfaces. he biomechanical properties of these structures and of the vaginal wall