REGULATION OF THE SWITCH FROM MYOMETRIAL CONTRACTURES TO CONTRACTIONSIN LATE PREGNANCY - STUDIES IN THE PREGNANT SHEEP AND MONKEY

Myometrial contractility occurs throughout pregnancy and characteristi c patterns of myometrial activity exist according to the endocrine sta tus 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 contracti on-type activity at term. This switch from contractures to contraction s tends to occur at night and is related to alteration in maternal pla sma oestrogen concentrations, and maternal oxytocin function. Studies in several animal species support the hypothesis that maternal oestrog en prepares the myometrium for a periodic signal that causes the switc h from contractures to contractions. Several lines of evidence implica te oxytocin in the switch. These studies show that the detailed prepar ation for parturition takes longer than previously considered and is b rought about by a carefully regulated sequence of events in which oest rogen production by the placenta plays a central role.