Estimating the effectiveness of emergency contraceptive pills

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...

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Bibliographic Details
Published in:Contraception (Stoneham) 2003-04, Vol.67 (4), p.259-265
Main Authors: Trussell, James, Ellertson, Charlotte, von Hertzen, Helena, Bigrigg, Allison, Webb, Anne, Evans, Margaret, Ferden, Sue, Leadbetter, Clare
Format: Article
Language:English
Subjects:
Adult
Biological and medical sciences
Birth control
Clinical Trials as Topic
Coitus - physiology
Contraceptives, Postcoital, Hormonal - economics
Contraceptives, Postcoital, Hormonal - therapeutic use
Cost-Benefit Analysis
Effectiveness
Emergencies
Emergency contraception
Female
Gynecology. Andrology. Obstetrics
Hormonal contraception
Humans
Medical sciences
Menstrual Cycle - physiology
Ovulation - physiology
Pregnancy
Statistics as Topic
Time Factors
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Summary: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. 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. 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. 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.
ISSN:0010-7824
1879-0518
DOI:10.1016/S0010-7824(02)00535-8