Delayed endometrial maturation induced by daily administration of the antiprogestin RU 486: A potential new contraceptive strategy

Objective: Our objective was to determine if a progesterone antagonist might interdict the development of a secretory endometrium. Study Design: Eleven normally cycling women not at risk for pregnancy received RU 486 (1 mg/day orally) or placebo throughout one menstrual cycle in a randomized, double-blind, crossover fashion. Estradiol, progesterone, and placental protein 14 were measured every 3 days; luteinizing hormone was measured until the midcycle surge was detected. An endometrial biopsy was performed on luteal phase day 7 to 9 and interpreted with Noyes' criteria. Differences between treatment groups were analyzed by the Student t test. Results: RU 486 delayed ovulation, retarded endometrial maturation, and reduced peak levels of placental protein 14 without affecting gonadal steroid production. The abnormalities in endometrial morphology and function are similar to those seen in infertile women with luteal phase defects. Conclusion: We hypothesize that this regimen of antiprogestin administration may prevent implantation and offer a novel strategy for fertility control.