The initiation of parturition: A comparative analysis across the species

Considerable evidence suggests that the fetus in most viviparous animals is in control of its own destiny with regard to the timing of labor and, thus, its birth. Exactly how the fetus triggers labor, however, is still unknown in most species. It is known that the mechanisms responsible for the initiation of parturition at term differ significantly among the species. Such differences may reflect the evolutionary status of the organism in question or may represent innovative solutions to inherent obstacles to reproduction faced by each species (eg, differences in placentation, differences in gestational length, or differences in the number of offspring per pregnancy). Pregnancy in certain species, including humans, is maintained by the fetoplacental unit, and labor results from the removal of mechanisms that maintain uterine quiescence and from the recruitment of interactive positive feed-forward loops that promote uterine activity. In other species, pregnancy is maintained by continued production of progesterone by extrauterine tissues (the ovary in mice and, possibly, the mammary gland in goats). In such animals, progesterone withdrawal is a prerequisite for labor. This monograph reviews in detail our current understanding of the mechanisms involved in the initiation of labor at term in a number of species, with particular attention being given to human pregnancy and labor. A better understanding of such mechanisms will further our knowledge about parturition disorders, including preterm labor and failed post-term induction of labor.