Parturition

The success of pregnancy depends on the appropriate process and timing of parturition. For each viviparous species, natural selection has affected extant physiology such that birth processes and timing complement overall reproductive strategy to maximize reproductive efficiency. This chapter synthesizes the current understanding of the physiology and pathophysiology of human parturition from the perspective of the evolutionary biology and comparative physiology of birth timing. We examine selective pressures that may have influenced parturition physiology and pathophysiology; the principal organ for parturition, the uterus, and its functional tissue types: the myometrium, cervix, and decidua; and the hormonal interactions that control their function to either sustain pregnancy or facilitate labor and delivery. Conserved uterotropic (causing uterine growth) and uterotonic (causing uterine contraction) hormonal pathways are present across viviparous species, although their regulatory patterns and sites of production differ. An emerging theme is that parturition involves positive-feedback loops leading to intrauterine inflammation, enhanced uterine contractility, membrane rupture, and cervical dilation, functioning at a paracrine level within each of the gestational tissues and in an endocrine fashion between the uterus and extrauterine sites.