The placental transfer of mifepristone (RU 486) during the second trimester and its influence upon maternal and fetal steroid concentrations.

A double-blind, placebo-controlled study has assessed the maternal and fetal endocrine effects of the maternal administration of the anti-progestin mifepristone in mid-pregnancy. There were six women in each group. Four hours after oral administration of 600 mg mifepristone, the drug was detected in both maternal and fetal circulations and in the amniotic fluid. No significant changes in progesterone, cortisol, oestradiol, or aldosterone concentrations were detected in the maternal circulation after treatment with mifepristone or placebo. In women treated with mifepristone, the mean fetal aldosterone level was 1699 (SD 217) pmol/l which was significantly higher than the mean level of 999 (SD 84) pmol/l in the control group but no significant changes occurred in the fetal progesterone, oestradiol or cortisol concentrations. The significance of these results is discussed in relation to the possible therapeutic uses of mifepristone for inducing labour.