Dr. Valenzuela proposes quantitative estimation of postovulatory or postfertilization effects of levonorgestrel emergency contraception (LNG EC) based on calculating the maximum possible effectiveness that can be expected from preovulatory effects with different duration of delay between intercourse and administration of LNG EC. We believe this approach is sound and provides important insight into the potential effectiveness and mechanisms of action of LNG EC. In our original comment we expressed this idea in a qualitative way; in the meantime we have completed a quantitative estimation that comes to similar conclusions as those presented by Dr. Valenzuela. However in Dr. Valenzuela?s brief description the rationale for multiplying daily fecundity by equal probability of having intercourse for all days of the cycle is not clear. We think it is sufficient to use the original estimates of day-specific probability of pregnancy. It must be noted however that the cited estimates from the study byWilcox et al. include biochemical pregnancies which are not likely to be detected in most clinical trials of EC. Estimates restricted to clinical pregnancies were calculated for the same data by Trussell et al. Also the weighting by 0.7 or 0.3 of the two different data sets for day-specific probability of pregnancy is a rather crude way of combining data leading to an improved estimation only when both samples are assumed to be equally different from the true estimate. In fact there are good reasons to suggest that fecundity in users of natural family planning is higher than in the general population. Despite these methodological issues we agree with Dr. Valenzuela that any observed effectiveness over 50% with more than 24 h delay of administration is strong evidence for postfertilization mechanisms of LNG EC and/or presents a substantial overestimate of the effectiveness of LNG EC. We further agree that ?the critical comparison is the decay pattern of expected and observed effectiveness in relation tothe time between intercourse and drug administration.? (full text)