Objective: To consider the risk of intercourse without contraception during infertility treatment. Design: Case report. Setting: Leiden University Medical Center. Patient(s): An infertile couple... Show moreObjective: To consider the risk of intercourse without contraception during infertility treatment. Design: Case report. Setting: Leiden University Medical Center. Patient(s): An infertile couple underwent IVF for tubal pathology. Intervention(s): Transfer of one embryo during a natural cycle. Main Outcome Measure(s): Human leukocyte antigen typing, histochemical analysis of the fetal membranes, neonatal and maternal clinical outcomes. Result(s): A dizygotic twin pregnancy was confirmed after birth by human leukocyte antigen typing of both fetuses and mother and by histochemical analysis of the dividing fetal membranes. This suggests a pregnancy of concurrent IVF and spontaneous conception. Pregnancy was complicated by preeclampsia and intrauterine growth retardation of both fetuses. Conclusion(s): We state that couples should abstain from intercourse without contraception during infertility treatment to prevent multiple gestation and its related complications for mother and fetuses. (Fertil Steril (R) 2011;95:805.e1-e3. (C)2011 by American Society for Reproductive Medicine.) Show less