Mechanisms of action of emergency contraception.

The use of levonorgestrel (LNG) alone or combined with ethinylestradiol (Yuzpe regimen), for hormonal emergency contraception (HEC) has been approved in several countries whereas in others it is still under debate or has been rejected under the claim that these formulations abort the developmental potential of the embryo. The issue is whether they act by preventing fertilization or by impeding the successful development of the zygote through and beyond implantation. Until now, published work has left this issue largely unresolved, and this paucity of knowledge sustains heated controversies in many settings. A single study indicates that LNG impairs sperm migration in the genital tract of women in ways that could interfere with fertilization. Several studies in women examined the effects of HEC on the outcome of the leading follicle, but lack of precision in the timing of treatment relative to follicular growth, maturation, or rupture confers great variability and inconsistency of results within and between studies. Nonetheless, results indicate that ovulatory dysfunction may account for the prevention of pregnancy in a large proportion of cases. Studies searching for possible alterations of the endometrium at the time implantation would normally take place, found minimal changes of doubtful significance. Recent studies in animals cast serious doubts that LNG prevents pregnancy by interfering with post-fertilization events. Failure to prevent expected pregnancies is close to 25% in women, and this is likely to be accounted for entirely by treatment given too late to prevent fertilization. The exact mode of action of HEC remains undetermined.