NEW ESTIMATES OF THE EFFECTIVENESS OF THE YUZPE REGIMEN OF EMERGENCY CONTRACEPTION

The purpose of this study was to provide new estimates-of the effectiv eness of the Yuzpe method of emergency contraception and to offer corr ectly computed estimates of the confidence intervals for estimated eff ectiveness rates. Through a literature search, seven studies that pres ent the number of women treated and outcome of treatment by cycle day of unprotected intercourse relative to expected day of ovulation were identified. Probabilities of conception by cycle day of intercourse am ong women not using contraception and the associated variance-covarian ce matrix from five other datasets were.estimated, and these external estimates were used to assess the effectiveness of the Yuzpe regimen. The 40 estimates of effectiveness, based on seven separate studies and the seven studies combined and five different sets of conception prob abilities by cycle day, ranged from a low of 44.2% to a high of 88.7%. The preferred point estimate is that emergency contraceptive pills re duce the risk of pregnancy by 75.4%, with a 95% confidence interval ex tending from 65.6% to 82.4%. True effectiveness is Likely to be at lea st 75% because treatment failures (observed pregnancies) include women who were already pregnant when treated and women who became pregnant after being treated. CONTRACEPTION 1998;57:363-369 (C) 1998 Elsevier S cience Inc. All rights reserved.