FIGURE 21-16 Diminution in size of the placental site after birth of the newborn. A. Spatial relations before birth. B. Placental spatial relations after birth.
the neonate is born, the uterus spontaneously contracts around than the cause of the separation, because in some cases bleeding is negligible.
FIGURE 21-17 Postpartum, membranes are thrown up into folds as the uterine cavity decreases in size. (Used with permission from Dr. Kelley S. Carrick.)
The great decline in uterine cavity surface area simultaneously throws the fetal membranes-the amniochorion and the parietal decidua-into innumerable folds (Fig. 21-17). Membranes usually remain in situ until placental separation is nearly completed. These are then peeled of the uterine wall, partly by further contraction of the myometrium and partly by traction that is exerted by the separated placenta as it descends during expulsion.
After the placenta has detached, it can 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. 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 irst 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 irst.
Late in pregnancy, during phase 2 of parturition, the number of myometrial oxytocin receptors grows appreciably (Fuchs, 1982; Kimura, 1996). his increase coincides with a greater uterine contractile responsiveness to oxytocin. Prolonged gestation is associated with a delay in the rise of these receptor levels (Fuchs, 1984).
Oxytocin-literally, quick birt-was the irst uterotonin to be implicated in parturition initiation. his 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).
In addition to its efectiveness in pharmacologically 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 rises 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).