Labor (parturition) refers to the series of events that expel the fetus out of the mother's body by means of uterine contractions. The musculature of the pregnant uterus is arranged in three strata: an external hood-like layer, which arches over the fundus and extends into the various ligaments; an internal layer consisting of sphincter-like fibers around the orifices of the tubes and the internal Os; and the main portion of the uterine wall, formed by a middle layer, which consists of an interlacing network of muscle fibers. The uterus has pacemakers to produce the rhythmic coordinated contractions of labor.

Parturition results from a combination of several different factors that progressively increase the excitability of the uterine musculature. The placenta at term begins to secrete a higher ratio of estrogen (which excites uterine activity) to progesterone (which inhibits uterine activity). The fetal size produces a stretching of uterine musculature...