Biology · Ch 2 — Human Reproduction
Parturition — Neuroendocrine Mechanism
Parturition — Neuroendocrine Mechanism
Parturition, or childbirth, is the process by which the fully developed foetus, together with the placenta, is expelled from the mother's body, normally occurring at the end of a gestation period of about nine calendar months (approximately 40 weeks from the mother's last menstrual period). Throughout most of pregnancy, the uterus remains relatively quiescent, its powerful muscular wall (the myometrium) kept from contracting strongly by the high levels of progesterone secreted first by the corpus luteum and later by the placenta itself; parturition requires that this quiescence be overcome and replaced by strong, coordinated, rhythmic uterine contractions.
The precise trigger for the onset of labour is understood to originate mainly from signals generated by the fully developed foetus itself, together with changing hormonal conditions in the mother, and is described as a neuroendocrine mechanism because it links mechanical/neural signals (uterine stretch, sensory input) to hormonal release. As the foetus reaches full term, mild and irregular contractions of the uterus (sometimes called Braxton-Hicks-type contractions) increase, and pressure exerted by the foetus's head against the cervix generates sensory signals that are relayed to the mother's hypothalamus. These signals stimulate the release of the hormone oxytocin from the posterior pituitary; oxytocin acts directly on the smooth muscle of the myometrium, stimulating it to contract more strongly, and it also stimulates the placenta to release prostaglandins, which further intensify uterine contractility. …
Worked out. Most hormonal control loops in the body work by negative feedback, damping a signal down once enough of it is present, but parturition is a rare example of a hormonal process driven instead by positive feedback, in which a rising signal drives even more of the same signal rather than suppressing it. Mild, irregular uterine contractions late in pregnancy, together with pressure signals from the fully developed foetus's head against the cervix, are believed to trigger the release of a small amount of oxytocin from the mother's posterior pituitary; this oxytocin further stimulates uterine contraction and increases stretching of the cervix, which in turn stimulates the release of even more oxytocin. This mutually reinforcing cycle of stretch and oxytocin release, called the foetal ejection reflex, escalates the strength and frequency of uterine contractions until they are powerful and coordinated enough to expel the foetus through the birth canal, and the loop is only broken once delivery h …