Progesterone withdrawal and parturition.

The steroid hormone progesterone (P4), acting via the nuclear P4 receptors (PRs) in uterine cells, is essential for the establishment and maintenance of pregnancy. P4/PR signaling maintains pregnancy by promoting uterine quiescence and blocking the contractions of labor. Withdrawal of the P4/PR block to labor induces parturition. The success of pregnancy requires the timely birth of a mature neonate to a healthy mother, and to this end, the mechanism by which the P4/PR block is withdrawn, and how that process is physiologically controlled is critical. This review examines current understanding of cell and molecular biology of P4/PR withdrawal in the control of parturition.