Other fetal abnormalities that prevent or severely reduce the entry of fetal urine or lung secretions into amnionic fluid do not prolong human pregnancy. Examples are renal agenesis and pulmonary hypoplasia, respectively. 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. Indeed, 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 Cwsteroid hormones (Chap. 5, p. 104). Such pregnancies are associated with delayed labor and suggest that the fetal adrenal glands are important for the timely onset of parturition.
PHASE 3: 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 21-4. he irst stage begins when spaced uterine contractions of suicient frequency, intensity, and duration are attained to bring about cervical thinning, termed facement. Several uterotonins may be important to the success of this stage of active labor (see Fig. 21-5). These have been shown to stimulate smooth muscle contraction through G-protein coupling. This labor stage ends when the cervix is fully dilated-about 10 cm-to allow passage of the term-sized fetus. The irst stage of labor, therefore, is the stage of cervical eacement and dilation. The second stage begins when cervical dilation is complete and ends with delivery. Thus, the second stage of labor is the stage of etal 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.
Body Anat.eI.O. Hist.eI.O. Active segmentI II E.O. � Pathological retraction ring (Sandi) Obliterated 1.0. Active segmentIII Passive segment PREGNANT UTERUS AT TERM UTERUS IN LABOR NORMAL EARLY FIRST STAGE UTERUS IN LABOR NORMAL SECOND STAGE UTERUS IN LABOR ABNORMAL SECOND STAGE -DYSTOCIA FIGURE 21-11 Sequence ofdevelopment ofthe 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 ofthe upper and lower uterine segments. The pathological retraction ring develops from the physiological ring. Anat. 1.0. = anatomical internal os; E.O. = external os; Hist. 1.0. = histological internal os;
Ph.R.R. physiological retraction ring.
• First Stage: Clinical Onset of Labor