Endocrinology of Parturition 1

Abstract

To insure optimal survival of the neonate, the timing of parturition must be tightly controlled. Parturition before term dramatically increases the risk of the birth of a nonviable premature infant. At a time after the baby is mature, the intrauterine environment becomes hostile. Postdate fetuses have an increased incidence of intrauterine demise. To maximize infant survival, mechanisms have evolved to allow for uterine distention and reduced contractions during pregnancy and the timely initiation of labor. In women this process is only partially understood. The control of the timing of labor is complex and involves interactions of the mother, the fetus, and the placenta plus membranes. In women, invasive study of the intrauterine environment is limited for ethical reasons. However, the control of pregnancy and parturition is highly species specific. Although much more is known about the process of parturition in rodents and sheep than in humans, there are no good nonprimate models of human parturition. Even subhuman primates have significantly different endocrine environments and controls than humans. There is a complex schema of the initiation of labor in women, and there is no simple chain of events as there are in other species. For instance, there cannot be shown an acute rise in circulating estrogen, nor an abrupt decline in circulating progesterone levels to signal the onset of labor in all pregnancies. Evidence suggests that there are multiple paracrine/autocrine events, fetal hormonal changes, and overlapping maternal/fetal control mechanisms for the triggering of parturition in women.