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STATISTICAL EVIDENCE ABOUT THE MECHANISM OF ACTION OF THE YUZPE REGIMEN OF EMERGENCY CONTRACEPTION

Trussell, James PhD; Raymond, Elizabeth G. MD, MPH

Reviews

Objective To determine whether published statistical evidence about the effectiveness of the Yuzpe regimen of emergency contraception provides insight about its mechanism of action.

Data Sources We searched the literature for studies that present information on the effectiveness of the Yuzpe regimen, on the probability of conception by menstrual cycle day, or on the occurrence of ovulation in women treated with the regimen. Searches of the electronic databases MEDLINE, POPLINE, EMBASE, and BIOSIS were supplemented by scrutiny of the bibliographies of all papers identified through the electronic search.

Methods of Study Selection We identified a review of the effectiveness of the Yuzpe regimen based on all seven clinical trials that present the number of women treated on each cycle day and the outcome of each treatment; this review also provided external estimates of the probability of conception by cycle day of unprotected intercourse from two other clinical studies. We identified three clinical studies of ovulation after treatment with the Yuzpe regimen. We included all identified studies in our analysis.

Tabulation, Integration, and Results We compared 40 estimates of the actual effectiveness of the Yuzpe regimen with the maximum theoretical effectiveness that could be obtained if the regimen worked only by preventing or delaying ovulation. In the overwhelming majority of these comparisons, the former exceeded the latter.

Conclusion The Yuzpe regimen could not be as effective as it appears to be if it worked only by preventing or delaying ovulation.

Office of Population Research, Princeton University, Princeton, New Jersey; and the Biomedical Affairs Division, Family Health International, Research Triangle Park, North Carolina.

Address reprint requests to: James Trussell, PhD Office of Population Research Princeton University Notestein Hall Princeton, NJ 08544 E-mail: trussell@princeton.edu

The authors have no financial interest whatsoever in the commercial success or failure of the Yuzpe method of postcoital contraception. This work was supported by NIH grants R01HD34016 and P30 HD/AG32030.

Received April 13, 1998. Received in revised form June 22, 1998. Accepted July 17, 1998.

© 1999 The American College of Obstetricians and Gynecologists