Fetal-placental and decidual-placental units: role of endocrine and paracrine regulations in parturition.

In primates, fetal adrenal and placental steroidogenic enzymatic systems are complementary in a fetal-placental unit, synchronizing fetal maturation and myometrial activation in late gestation. Moreover, as hemochorial placentation characterizes rodents and primates, paracrine regulations between decidua and placenta are essential to the immunotolerance of the conceptus and its development. Thus, the decidual-placental unit remains in a striking state of decidual quiescence throughout gestation, and the reversal of this quiescence is thought to play a key role in myometrial stimulation and the onset of parturition. A comprehensive view of the control of myometrial contractility, through the interaction of paracrine and endocrine modifications in late gestation, is proposed. The failure of these mechanisms underlie prematurity and the use of fetal therapy in threatened preterm labor.