Comparative studies on the initiation of labor.

Parturition is a multifactorial physiological process that involves multiple interconnected positive feedforward and negative feedback loops. Each of these loops is connected to others in a carefully time-regulated fashion. When parturition occurs normally, both fetal and maternal processes are involved. Recent research in pregnant sheep and nonhuman primates shows clearly that the fetus determines the duration of gestation and the mother determines the time of day at which labor begins. The fetal hypothalamo-pituitary-adrenal axis plays a fundamental role in stimulating increased estrogen production in late gestation. Estrogen then activates maternal circadian rhythms that increase oxytocin secretion. As a result, uterine contraction patterns are switched from contractures to contractions.