OBJECTIVE: The study was undertaken to ascertain whether body mass index (BMI) affects first-trimester pregnancy outcome in patients with infertility. STUDY DESIGN: Records of 494 patients with a singleton gestation after treatment for infertility at a single academic center were retrospectively reviewed. Patients were classified with regard to BM I and treatment protocol. Outcomes were defined as ongoing pregnancy, spontaneous abortion, or ectopic pregnancy. RESULTS: The average rate of spontaneous abortion was 21.5% and of ongoing pregnancy 75.1%. This did not differ significantly in any of the BMI groups. When patients were further subdivided according to diagnosis or fertility treatment, the spontaneous abortion rate remained the same. CONCLUSION: The likelihood of a spontaneous abortion, ongoing pregnancy, or ectopic pregnancy in singleton gestations in the first trimester, after treatment for infertility, was not affected by BMI.