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While it is recommended that ECPs be taken within 72 hours of unprotected intercourse for maximum effectiveness, ECPs may have some residual effect beyond 72 hours, particularly if ovulation has not occurred.
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It is theorized that the efficacy of ECPs taken after 72 hours is lower than the efficacy of ECPs taken within the recommended window of 72 hours. Almost all studies, thus far, have only measured the effectiveness of ECPs up to 72 hours after intercourse. If the regimen is initiated more than 72 hours after intercourse, the failure rate may be increased.
- Webb A. When to use post-coital contraception. Fertility Control Reviews 1992;2(2):15-7.
- Emergency oral contraception. ACOG Practice Patterns 1996;3.
However, if the primary mechanism of action of ECPs is the prevention or delaying of ovulation, variations in timing of ECP use in relation to ovulation could result in ECPs being effective for longer than 72 hours. The 72 hour limit is currently being investigated.
- Swahn ML, Westlund P, Johannisson E, Bygdeman M. Effect of post-coital contraceptive methods on the endometrium and the menstrual cycle. Acta Obstetricia et Gynecologica Scandinavica 1996;75:738-44.
- Trussell J, Ellertson C, Rodriguez G. The Yuzpe regimen of emergency contraception: How long after the morning after? Obstetrics and Gynecology 1996;88:150-4.
- Grou F, Rodrigues, I. The morning-after pill-how long after? American Journal of Obstetrics and Gynecology 1994;171:1529-34.
- Consortium for Emergency Contraception. Emergency contraceptive pills update. Welcome, Maryland: The Consortium, March 1997.
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