Effects of sexual intercourse patterns in time to pregnancy studies.

Time to pregnancy, typically defined as the number of menstrual cycles required to achieve a clinical pregnancy, is widely used as a measure of couple fecundity in epidemiologic studies. Time to pregnancy studies seldom utilize detailed data on the timing and frequency of sexual intercourse and the timing of ovulation. However, the simulated models in this paper illustrate that intercourse behavior can have a large impact on time to pregnancy and, likewise, on fecundability ratios, especially under conditions of low intercourse frequency or low fecundity. Because intercourse patterns in the menstrual cycles may vary substantially among groups, it is important to consider the effects of sexual behavior. Where relevant and feasible, an assessment should be made of the timing and frequency of intercourse relative to ovulation. Day-specific probabilities of pregnancy can be used to account for the effects of intercourse patterns. Depending on the research hypothesis, intercourse patterns may be considered as a potential confounder, mediator, or outcome.