Effect of mifepristone on inhibition of ovulation and induction of luteolysis.

Mifepristone administration to women in the mid- or late follicular phase delays the luteinizing hormone (LH) surge and prolongs the follicular phase. Since the resumption of follicular growth commences following mifepristone cessation, the drug must be given either continuously or at repeated intervals in order to block ovulation. Using various regimens with or without exogenous progestins, ovulation was inhibited in the majority of subjects. However, this was not consistent and in several instances, LH surges and a rise in plasma progesterone were suggestive of ovulation and corpus luteum function. Therefore, mifepristone cannot be used as a contraceptive which will reliably inhibit ovulation. With low-dose mifepristone administration, alterations in endometrial morphology were characterized by a delay in maturation despite the presence of ovulation. This suggests that the endometrium displays a greater sensitivity to mifepristone than does the pituitary, a finding that may have important contraceptive implications. Late luteal-phase mifepristone administration to women who were not sexually active did not alter their menstrual rhythm, bleeding patterns or steroid and gonadotrophin concentrations. However, when used in unprotected women in the late luteal phase, mifepristone did not uniformly terminate all pregnancies. On the other hand, when used within 72 h of intercourse, mifepristone was as effective a post-coital agent as the standard high-dose oestrogen-progestin combination.