The purpose of this study was to provide new estimates of the effectiveness of the Yuzpe method of emergency contraception and to offer correctly computed estimates of the confidence intervals for estimated effectiveness rates. Through a literature search, seven studies that present 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 among women not using contraception and the associated variance-covariance 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 probabilities 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 reduce the risk of pregnancy by 75.4%, with a 95% confidence interval extending from 65.6% to 82.4%. True effectiveness is likely to be at least 75% because treatment failures (observed pregnancies) include women who were already pregnant when treated and women who became pregnant after being treated.
PIP: The Yuzpe method of emergency contraception involves 200 mcg of ethinyl estradiol and 2 mg of norgestrel taken in two doses--the first within 72 hours of unprotected intercourse and the second 12 hours after the first. An earlier review (1996) of 10 studies (some of which were methodologically flawed) estimated a pooled method effectiveness rate of 74%. The present literature search identified seven studies in which researchers separately reported both the number of women treated with the Yuzpe regimen and the resulting pregnancies by cycle day of intercourse relative to the expected day of ovulation. Probabilities of conception by cycle day of intercourse among women not using contraception and the associated variance-covariance matrix from five other data sets were estimated. The estimated effectiveness rates ranged from a low of 44.2% to a high of 88.7%. This rate was higher in studies that counted all conceptions, including those resulting in early pregnancy losses. An effectiveness rate of 75.4% (95% confidence interval, 65.6-82.4%) based on conception probabilities from the pooled data in which only recognizable conceptions were counted is considered the best single summary statistic. This new estimate suggests the Yuzpe regimen of emergency contraception is at least as effective as believed previously. Moreover, the true effectiveness is likely to exceed 75% inasmuch as the studies included some women already pregnant at the time of treatment and others who became pregnant from intercourse subsequent to treatment.