OBJECTIVE:To determine the impact of sexual intercourse around the time of implantation on the probability of achieving a pregnancy. DESIGN:Time-to-pregnancy cohort using day-specific probability of pregnancy modeling to account for intercourse during the fertile window. SETTING:Community cohort. PATIENT(S):Women trying to conceive naturally, ages 30-44, without known infertility. INTERVENTION(S):None. MAIN OUTCOME MEASURE(S):Positive pregnancy test. RESULT(S):A total of 564 women provided 1,332 complete cycles for analysis. Intercourse frequency during the fertile window and during the peri-implantation window were significantly correlated. Cycles in which couples had 2 or more days with intercourse during the implantation window were significantly less likely to result in a positive pregnancy test compared with cycles in which couples did not have intercourse in this window, after adjusting for age, race, history of regular menstrual cycles, previous pregnancy, and body mass index (fecundability ratio, 0.62; 95% confidence interval, 0.42-0.91). CONCLUSION(S):Intercourse during the peri-implantation window may be detrimental to natural fertility. Methods that allow couples to time intercourse to the fertile window may decrease time to pregnancy by not only increasing the probability of fertilization but also decreasing the probability of failed implantation.