Effectiveness of the Yuzpe regimen of emergency contraception by cycle day of intercourse: implications for mechanism of action

OBJECTIVE: The purpose of this study was to provide evidence about the mechanism of action of the Yuzpe method of emergency contraception by examining effectiveness by cycle day of intercourse relative to ovulation. METHODS: Through a literature search, we identified eight studies that present the number of women treated and outcome of treatment by cycle day of unprotected intercourse relative to expected day of ovulation. Using five sets of external estimates of conception probabilities by cycle day of intercourse among women not using contraception, we assessed and compared the effectiveness of the Yuzpe regimen by whether intercourse occurred on or before the second day before ovulation or afterward, and whether intercourse occurred on or before the first day before ovulation or afterward. RESULTS: In 36 of the 45 pairs of estimates of effectiveness, based on eight separate studies and the eight studies combined and five different sets of conception probabilities by cycle day, effectiveness was higher-and in most cases substantially higher-when intercourse occurred on or before the second day before ovulation (day -2) than when it occurred later. When data were stratified by whether intercourse occurred on or before the day before ovulation (day -1), effectiveness was greater when intercourse occurred early in 43 of 45 pairs. CONCLUSIONS: These results suggest that one hypothesized mechanism of action of the Yuzpe method, inhibiting implantation of a fertilized egg, is unlikely to be the primary mechanism of action.