D uring pregnancy, the muscular layer of the uterine wall known as the myometrium, which is composed mainly of smooth muscle cells, is maintained in a state of relative quiescence. A switch from myometrial quiescence to myometrial activation is required to establish uterine contractions during labor. Researchers have long been perplexed by the fact that the major prostaglandin produced by the uterus just prior to labor, prostacyclin, is a smooth muscle relaxant. In this issue of the JCI, Fetalvero et al. provide data that they propose explains this paradox, at least in part (see the related article beginning on page 3966). The authors examined uterine tissue from pregnant women near term and found that prostacyclin stimulation, which raises cAMP levels that were previously thought to affect only myometrial quiescence, can promote myometrial activation over time by increasing the expression of a select group of proteins thought to be indicative of a uterine contractile state.