Endocrine Control of Parturition

Abstract

In their introduction to the previous edition, Thorburn et al. (1977) stated: “We now recognize that in late pregnancy a train of events is initiated that ultimately results in the delivery of the fetus. However, we still do not know exactly how and where the train starts, or exactly how it exerts its ultimate action on the myometrial cell.” Little has changed in the last decade to increase our understanding of these events. Although the initiation of parturition is generally understood, the precise trigger for labor is still unknown. In addition, labor is complicated by different mechanisms in different species. For example, the onset of labor in rats and rabbits is rapid: uterine contractions become intense immediately before delivery, and the newborn are expelled rapidly. In humans, monkeys, and guinea pigs, labor develops slowly and is protracted. Schofield (1968) suggested that in species with a large fetus relative to the mother, a more protracted delivery may be an advantage. In the human and monkey, uterine motility evolves gradually during the last trimester of pregnancy, and actual labor often precedes delivery by many hours. It is possible that different mechanisms are at play in rapid-onset and protracted-onset types of labor.