Parturition, Labor, Delivery

The appropriate timing and process of parturition (birth) is essential for the success of pregnancy and involves contraction of the myometrium (smooth muscle of the uterine wall), softening and dilation of the uterine cervix, and weakening and rupture of the fetal membranes. The contractile state of the gravid uterus is controlled mainly by the steroid hormone progesterone, which blocks parturition by promoting myometrial relaxation, closure of the cervix, and fetal membrane integrity. Withdrawal of the progesterone block is a key trigger for parturition and is associated with marked uterine tissue-level inflammation. In women parturition is triggered by a functional desensitization of uterine cells to progesterone mediated by multiple mechanisms including altered transcriptional activity of the nuclear progesterone receptors (PRs). In myometrial cells PR-mediated functional progesterone withdrawal is induced by pro-inflammatory stimuli. This model explains the induction of parturition by stressors impacting the gravid uterus and may have evolved in response to the combined selective pressures of encephalization and bipedalism that favored a parturition trigger mechanism to ensure that the fetus is born before its head becomes too large to pass through the pelvic outlet.