Estimating the effectiveness of emergency contraceptive pills.

OBJECTIVE: We use new estimates of conception probabilities by cycle day of intercourse, where cycle day is measured with day 1 being the first day of bleeding in a cycle, to propose a new approach for estimating the effectiveness of emergency contraceptive pills (ECPs). We use this new approach to examine the absolute effectiveness and the cost-effectiveness of ECPs and whether ECPs are more effective the sooner after unprotected intercourse they are initiated. METHODS: Using the new set of conception probabilities, we employ data from two recent clinical trials of ECPs, one from the Population Council and the other from the World Health Organization (WHO), to examine the effectiveness of the combined ECP regimen. RESULTS: The expected pregnancy rate among typical users was 6.2% in the Population Council trial and 7.4% in the WHO trial based on conception probabilities by cycle day relative to the day of ovulation. Based on conception probabilities by cycle day relative to the first day of bleeding, the expected pregnancy rates dropped to 5.4% and 5.2%, respectively. The two trials yield conflicting evidence regarding whether effectiveness declines with treatment delay. CONCLUSIONS: Our results suggest that the absolute levels of effectiveness for the Yuzpe regimen of emergency contraception and the cost-effectiveness of this regimen have probably been overstated when based on conception probabilities by cycle day relative to day of ovulation.

Differences between emergency contraception users in the United States and the United Kingdom.

OBJECTIVES: to characterize emergency contraception (EC) users and clinical trial participants in the United States and the United Kingdom, comparing previous EC use and awareness, contraceptive history, and experience with EC. METHODS: We collected data from all EC seekers (n=5383) at 1 US and 2 UK clinics (9/97-8/98). We also collected detailed information from women (n=2157) enrolling in an EC trial at the first 3 clinics and 2 additional UK sites (9/97-2/00). RESULTS: More US (16%) than UK (4%) women reported additional acts (other than in the last 5 days) of unprotected sex during the cycle in which they sought EC. Fifty-eight percent of UK trial participants had used EC previously compared to 18% in the United States. Most participants in both groups used contraception regularly and reported needing EC because of condom breaks (67% and 56%). More UK than US participants used an ongoing method of contraception (38% v 28%). US women reported more side effects at follow-up than UK women did (76% v 59%), although similar proportions would take EC again or recommend it. CONCLUSIONS: US and UK women in our trial experienced different side effects. Researchers should use caution when presenting aggregate results from international multicenter trials. In addition, readers should be aware that such aggregate results might mask important geographical differences. More research on experience with EC and barriers to contraceptive use in the United States is needed.