Cortisol blockade of progesterone: a possible molecular mechanism involved in the initiation of human labor.

In most mammals, labor is heralded by progesterone withdrawal, which is believed to be related to the activation of multiple pathways leading to parturition. In humans, despite no decrease in placental progesterone production, activation of similar pathways preceding labor suggests the presence of an endogenous antiprogestin, which we reasoned might be cortisol, whose secretion from the fetal adrenal rises markedly at the end of human gestation. We report that in primary cultures of human placenta, cortisol is able to compete with the action of progesterone in the regulation of the corticotropin-releasing hormone (CRH) gene. CRH is a peptide highly expressed in human placenta at the end of gestation, which has been suggested to be involved in regulating the timing of parturition. These findings provide a model for functional progesterone withdrawal at the end of human pregnancy, which may be involved in the initiation of labor.