Biological Timekeeping During Pregnancy and the Role of Circadian Rhythms in Parturition

Abstract

A considerable body of evidence suggests that both the hour of onset of labor and the hour of birth in humans, as well as many animal species, is related to time of day. In humans, over 30 studies have demonstrated such a relationship, with the peak hours of onset of labor and of birth between 2300 and 0400 hours. In day-active animals a similar pattern exists, while in night-active species the reverse holds. Underlying the 24-h periodicity in onset of labor and birth time may be a rhythm in uterine myometrial activity. In rhesus macaques, uterine contractions show a 24-h rhythmicity with peak activity between 2300 and 0300 hours. The mechanism responsible for daily rhythms in uterine activity is proposed to reflect 24-h patterns of endocrine secretion. Several hormones in maternal and fetal circulation, including melatonin, cortisol (mother only), dehydroepiandrosterone (fetus only), and proges-terone, have also been shown to demonstrate 24-h rhythms, although not all these hormones have been studied in anyone species. Evidence in sheep demonstrates that the circadian melatonin pattern mediates the effect of photoperiod and controls the neuroendocrine system regulating reproduction. Extending this concept to the mechanism timing parturition, we raise the possibility that the 24-h melatonin pattern conveys information about photoperiod and synchronizes various maternal and fetal endocrine rhythms to the light-dark cycle. We hypothesize that the temporal coordination of these endocrine and uterine rhythms with environmental pho-toperiod plays a key role in the initiation of parturition.