Regulation of the switch from myometrial contractures to contractions in late pregnancy: studies in the pregnant sheep and monkey.

Myometrial contractility occurs throughout pregnancy and characteristic patterns of myometrial activity exist according to the endocrine status and the relationship to parturition. These characteristic patterns differ between species, yet certain common features can be observed. Throughout pregnancy, myometrial activity is of the contractures type, long-lasting, low-amplitude epochs of activity switching to contraction-type activity at term. This switch from contractures to contractions tends to occur at night and is related to alteration in maternal plasma oestrogen concentrations, and maternal oxytocin function. Studies in several animal species support the hypothesis that maternal oestrogen prepares the myometrium for a periodic signal that causes the switch from contractures to contractions. Several lines of evidence implicate oxytocin in the switch. These studies show that the detailed preparation for parturition takes longer than previously considered and is brought about by a carefully regulated sequence of events in which oestrogen production by the placenta plays a central role.