The primary mechanism of oral contraceptives is to inhibit ovulation, but this mechanism is not always operative. When breakthrough ovulation occurs, then secondary mechanisms operate to prevent clinically recognized pregnancy. These secondary mechanisms may occur either before or after fertilization. Postfertilization effects would be problematic for some patients, who may desire information about this possibility. This article evaluates the available evidence for the postfertilization effects of oral contraceptives and concludes that good evidence exists to support the hypothesis that the effectiveness of oral contraceptives depends to some degree on postfertilization effects. However, there are insufficient data to quantitate the relative contribution of postfertilization effects. Despite the lack of quantitative data, the principles of informed consent suggest that patients who may object to any postfertilization loss should be made aware of this information so that they can give fully informed consent for the use of oral contraceptives.