BACKGROUND: This study was conducted to evaluate the effects of levonorgestrel administration for emergency contraception (EC) on bleeding pattern and pituitary-ovarian function. STUDY DESIGN: In 69 women with a reported stable menstrual cycle length of 24-34 days, we investigated bleeding patterns following EC administration in the follicular (n=26), periovulatory (n=14) and luteal (n=29) phase. In a subgroup of 8 women, hormonal evaluation and ultrasonography were performed. RESULTS: EC taken in the follicular, but not in the periovulatory or luteal phase, significantly shortened cycle length by 10.9+/-1 days. The subsequent cycle was not affected. EC taken in the late preovulatory phase, prior to the gonadotrophin surge, suppressed ovulation (n=7), while ovulation was not blocked when EC was given during an ongoing luteinizing hormone (LH) pulse (n=1). CONCLUSIONS: Our data indicate that EC given before the onset of the luteinizing hormone (LH) surge inhibits ovulation and hastens the end of the current menstrual cycle. Subsequently, the length of the following menstrual cycle returned as prior to treatment. By contrast, levonorgestrel administered after the expected ovulation has no effect on menstrual cycle length.
