A B c Internal cervical as External cervical os \ Internal cervical os cervical os cervical os External cervical os FIGURE 21-14 Hydrostatic action of membranes in effecting cervical effacement and dilation, 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,
Physiology of Labor 415 its diminishing contents. Normally, by the time the newborn is completely deliv ered, the uterine cavity is nearly obliter ated. he organ consists of an almost solid mass of muscle, several centimeters thick, above the thinner lower segment. he uterine fundus now lies just below the level of the umbilicus .
in the area of the placental implantation 5.�2 c0 site (Fig. 21-16). For the placenta to accommodate itself to this reduced area, it thickens, but because of limited placen tal elasticity, it is forced to buckle. The resulting tension pulls the weakest layer decidua spongiosa-from that site. Thus, FIGURE 21-15 Labor course divided on the basis of expected evolution of the dilatation placental separation follows the disproand descent curves into three functional divisions. The preparatory division includes the portion created between the relatively latent and acceleration phases. The dilatational division is the phase of maximum slope of unchanged placental size and the reduced dilatation. The pelvic division encompasses both the deceleration phase and the second implantation site size.
stage, which is concurrent with the phase of maximum slope of fetal descent. (Redrawm
Cleavage of the placenta is aided from Friedman EA: Labor: Clinical Evaluation and Management, 2nd ed. New York, greatly by the loose structure of the
Appleton-Century-Crofts, 1978.) spongy decidua. As detachment proceeds, a hematoma forms between the separating change markedly during parturition. These result from altered placenta and the adjacent decidua, which remains attached to extracellular matrix structure or composition (Alperin, 2015; the myometrium. The hematoma is usually the result rather Rahn, 2008; Lowder, 2007).
The levator ani muscle closes 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 Boor, which are not spanned by the levator ani muscle, 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. he most marked change consists of stretching levator ani muscle fibers. This is accompanied by thinning of the central portion of the perineum, which becomes transformed from a wedgeshaped, 5-cm-thick tissue mass to a thin, almost transparent membranous structure less than 1 cm thick. hen 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: Delivery of Placenta and Membranes
This stage begins immediately after fetal delivery and involves separation and expulsion of the placenta and membranes. As