Parturition
Parturition occurs in three phases: (1) initiation of uterine contractions, (2) expulsion of the fetus, and (3) expulsion of the fetal membranes. However, events leading to birth result from a cascade of actions.
During most of gestation, maintenance of high progesterone concentrations maintain a "quiet" uterus; as birth approaches, Iuteolysis of the dominant CL and/or decreasing placental production of progesterone couples with increasing estrogen concentrations to prepare for parturition. Increasing estrogen also stimulates expression of oxytocin receptors in the myometrium. Activation of the fetal pituitary-adrenal axis is essential for the initiation of parturition. It is generally accepted that the fetus essentially becomes "stressed" so that it begins to secrete ACTH and therefore glucocorticoids. High concentrations act to remove the progesterone block by stimulation of three enzymes in the placenta that increase the conversion of progesterone to estradiol. These enzymes are 17a hydrolyase (conversion of progesterone to 17a hydroxyprogesterone), 17, 20 lyase (conversion of 17a hydroxyprogesterone to androstenedione), and aromatase (conversion of androstenedione to estradiol). Fetal corticoids also promote synthesis of PGF2oc, which also acts to remove the progesterone block. Increasing estradiol and PGF2OC Synergize to make the uterus more sensitive to actions that promote contractions. In some species (mares, human, rabbits, pigs) the hormone relaxin, which is produced by the ovary and/or placenta, acts to soften cervical tissues and allow pelvic ligament stretch to aid parturition. Interestingly, there is also growing evidence that relaxin and perhaps other milk-born factors are involved in essentially "programming" reproductive tract development in the neonate, that is, the Iac- tocrine hypothesis (see Chapter 18).As estradiol and glucocorticoids increase, the contracting uterus pushes the fetus toward the cervix. Neural signals from the cervix promote oxytocin release, more contractions, more pressure on the cervix, and more oxytocin in a positive cascade. As the fetus enters the cervical canal, the first stage of parturition is complete. Although estradiol and other hormones increase mucus secretion and general lubrication of the cervical canal and vagina, progressively stronger contractions are critical. With the fetus properly positioned, the feet and head put increasing pressure on the fetal membranes so that the membranes rupture, releasing amniotic fluid. This also acts as lubrication. As the fetus passes the birth canal it becomes hypoxic. This promotes fetal movement, which stimulates more uterine contraction. Along with active abdominal contractions of the dam, expulsion of the fetus is accomplished.
In most cases, the fetal membranes are expelled just after the fetus. The degree of disruption varies depending on the type of placentation but requires that the chorionic villi are removed from the crypts on the maternal side of the placenta. This believed to be caused by active vasoconstriction of the arteries of the villi. This is important to minimize hemorrhage. Clearly, this is also much more likely in animals with hemochorial or hemoendothelial types of placentation.
The duration of parturition varies between species. For example, in cows, initial stages of parturition (start of contractions and cervical dilation) typically last 2-6 hours, fetal expulsion 30-60 minutes, and expulsion of fetal membranes 6-12 hours. If the duration of parturition extends beyond the normal range for the animal this is called dystocia. One cause of problems is excessive size of the fetus. Consequently, care must be taken in breeding programs. Another problem occurs when there is a failure of the fetus to become properly positioned for birth. For example, in cattle, the calf is normally positioned as if it is diving head first with the front legs extended underneath the lower jaw. In about 5% of births, the calf is abnormally positioned. In some cases, normal birth becomes impossible so that a Cesarean section is required. A third issue arises in normally monotocous species when there are multiple births. This can be the result of twins presenting simultaneously, one abnormally positioned fetus blocking birth of another, or uterine fatigue from prolonged parturition.