Antifertility actions of the progesterone antagonist RU 486 include direct inhibition of placental hormone secretion.

the antifertility effects of the potent antiprogestin RU 486 (mifepristone) during early pregnancy have been attributed to its blockade of progesterone receptors in the endometrium. Studies in cultured syncytiotrophoblasts have revealed an additional action of RU 486 at the placental level, where it impairs the production of human chorionic gonadotropin (hCG), human placental lactogen (hPL), and progesterone. RU 486 (10 nmol-10 mumol/l) attenuated the production of all three placental hormones, in a dose-related manner, and its effects on hCG and hPL were reversed by addition of exogenous progesterone. The specific inhibitory effects of RU 486 on placental hormone secretion indicate that its antifertility actions are attributable to competitive inhibition of progesterone action in the trophoblast as well as in the endometrium.