Objective: To assess the possibility of a postfertilization effect in regard to the most common types of hormonal emergency contraception (EC) used in the US and to explore the ethical impact of this possibility.
Data sources and study selection: A MEDLINE search (1966-November 2001) was done to identify all pertinent English-language journal articles. A review of reference sections of the major review articles was performed to identify additional articles. Search terms included emergency contraception, postcoital contraception, postfertilization effect, Yuzpe regimen, levonorgestrel, mechanism of action, Plan B.
Data synthesis: The 2 most common types of hormonal EC used in the US are the Yuzpe regimen (high-dose ethinyl estradiol with high-dose levonorgestrel) and Plan B (high-dose levonorgestrel alone). Although both methods sometimes stop ovulation, they may also act by reducing the probability of implantation, due to their adverse effect on the endometrium (a postfertilization effect). The available evidence for a postfertilization effect is moderately strong, whether hormonal EC is used in the preovulatory, ovulatory, or postovulatory phase of the menstrual cycle.
Conclusions: Based on the present theoretical and empirical evidence, both the Yuzpe regimen and Plan B likely act at times by causing a postfertilization effect, regardless of when in the menstrual cycle they are used. These findings have potential implications in such areas as informed consent, emergency department protocols, and conscience clauses.