Parturition

Parturition (labor), the act of giving birth to young, marks the termination of pregnancy. Parturition may be divided into three stages. The first stage consists of uterine contractions that gradually force the fetus and fetal mem­brane to the cervix.

In the second stage, actual delivery of the fetus occurs. Passage of parts of the fetus through the cervix into the vagina along with rupture of one or both water bags reflexively initiates actual straining or contraction of the abdominal muscles. The combination of uterine contraction and abdominal contraction forces the fetus through the birth canal.

The third stage of parturition consists of delivery of the placenta, which normally follows the fetus almost immediately.

Late Gestation

To facilitate delivery of the fetus, muscles and ligaments of the birth canal relax shortly before parturition. The vulva swells, and a mucus dis­charge may be present. Muscles on both sides of the tail head may relax and appear lowered or depressed. Mammary glands enlarge and may secrete a milky material for a few days prior to parturition. As the time of parturition becomes imminent, animals may become rest­less, seek seclusion, and increase the frequency of attempts to urinate. The bitch and sow often try to build a nest.

An important endocrine change during late gestation in most species is in the ratio of estro­gen to progesterone. Progesterone is high rela­tive to estrogen during most of gestation, but this ratio changes during late gestation, with estrogen increasing relative to progesterone. Estrogen promotes the development of con­tractile proteins in the smooth muscle cells of the uterus and gap junctions between these cells. These uterine changes increase the force that the uterus can generate for delivery.

Initiation of Parturition

In domestic animals, an endocrine signal from the fetus appears to initiate parturition. Plasma levels of glucocorticoids (e.g., cortisol) increase in most domestic species, except for the mare, shortly prior to parturition. The fetal adrenal cortex is the source of these glucocorticoids, and the adrenal cortical secretion is in response to adrenocorticotropic hormone (AcTH) from the fetal adenohypophysis. An increase in sen­sitivity of the adrenal cortex to AcTH is part of the reason for the increased secretion of gluco­corticoids during this period.

The rising fetal glucocorticoids affect the placenta and the maternal uterus. in some species, the glucocorticoids increase estrogen production by the placenta so that plasma estrogens increase relative to progesterone. This brings about the changes in the uterine smooth muscle described earlier. Glucocorticoids and estrogens act synergistically to promote uterine synthesis and secretion of PGF _2α.

Increases in PGF_2a have multiple effects that can contribute to the onset of parturition, and some view these as the final link in the chain of events initiating parturition. in species whose primary corpus luteum remains and is a signifi­cant source of progesterone (e.g., cow and sow), the rise in PGF_2a can bring about leutoly- sis and remove this source of progesterone. Recall that progesterone suppresses the activity of uterine smooth muscle and maintains cervi­cal tone, so a reduction in progesterone is appropriate to facilitate delivery. PGF_2a also directly stimulates the contraction of uterine smooth muscle to move the fetus into the birth canal and directly promotes dilation of the cervix.

Oxytocin

The entrance of a fetus into the birth canal brings about a reflex increase in oxytocin secre­tion from the neurohypophysis. oxytocin acts directly on uterine smooth muscle to enhance uterine contractions and promote delivery. Extracts of the neurohypophysis are used clini­cally to stimulate contractions of the fatigued uterus during prolonged labor.

in the mare, plasma levels of oxytocin gradually rise during the latter stages of gesta­tion and then increase greatly as parturition begins.