Maternal-fetal conflict and the timing of birth.

Evolutionary theory predicts mammalian offspring will favor greater investment than parents are willing to provide, leading to conflict over resource transfer. This theory of parent-offspring conflict has been applied to resource transfer across the placenta. Birthweight and gestation length are functionally linked, suggesting that the timing of birth might also represent a focal point of maternal-fetal conflict. This hypothesis relies on two assumptions: mother and offspring have different fitness optima and both parties exert partial control over the timing of birth. It is argued, and evidence is reviewed, that suggests offspring benefit from slightly longer gestation than the maternal optimum, and that fetal and maternal genes both influence gestation length. We might therefore expect an evolutionary history of maternal-fetal conflict over the control of parturition. Evidence consistent with this hypothesis includes the effect of imprinted genes, as revealed through imprinting disorders, on gestation length; the wide variability in parturition mechanism(s) among mammalian species; and the complex physiology of human parturition including initiation by multiple different pathways with inputs from both mother and offspring. One potential consequence of a history of maternal-fetal conflict over control of the mechanisms of parturition is that the timing of birth may be subject to a greater degree of dysregulation than simpler physiological systems subject to single-party control.