Objective: To test the feasibility of timing the administration of mifepristone as a once-a-month contraceptive pill on the 12th day before the next menses, as calculated from the length of the previous menstrual cycles.
Design: Double-blind, randomized, controlled trial.
Setting: Five family planning centers across the world.
Patient(s): Three hundred ninety-nine women attending family planning clinics.
Intervention(s): Randomized to receive 10, 25, or 200 mg of mifepristone or a placebo.
Main outcome measure(s): Lengthening or shortening of the normal menstrual cycle length following administration of the drug by at least 5 days.
Result(s): The menstrual period came within 5 days of the predicted date in 88% of women receiving the placebo, 84% of women receiving 10 mg, 72% of women receiving 25 mg of mifepristone, and only 48% of women treated with 200 mg of mifepristone. Increasing the dose of mifepristone was associated with an increased chance of having a delayed period (P<.001). Only 45% of women were in the peri-ovulatory phase of the cycle according to LH and P measurements on the day of drug administration. Women treated before ovulation were more likely to have delayed menses with all three doses of mifepristone.
Conclusion(s): Because of the disruption in cycle length, it appears unlikely that mifepristone administered once a month, at a calendar-based time, would provide a reliable method of contraception.