Objective: To analyze the incidence of ectopic pregnancy (EP) in fresh compared with frozen-thawed cycles. Design: Retrospective cohort study. Setting: Teaching hospital. Patient(s): Thirty-one thousand nine hundred twenty-five women undergoing in vitro fertilization-embryo transfer (IVF-ET) from January 2006 to December 2013. Intervention(s): Fresh IVF-ET compared with frozen-thawed ET (FET). Main Outcome Measure(s): Incidence of EP with fresh IVF-ET compared with frozen-thawed ET cycles, clinical pregnancy rate, and rate of EP per clinical pregnancy. Result(s): For the fresh IVF cycles, 19,173 patients underwent oocyte retrieval; 15,042 had an ET, 6,431 of these patients (42.7%) had a clinical pregnancy, and among these 297 (1.97%) appeared to have an EP. The group of patients undergoing frozen-thawed ET (12,752 patients) included 12,255; there were 5,564 pregnancies (45.4%) and 124 ectopic implants (1.01%). The incidence of an EP per clinical pregnancy was 4.62% for the fresh transfer group compared with 2.22% for the frozen-thawed cycle group; this difference was statistically significant. In addition, the fresh ET cycles had the highest risk of EP, followed by day-3 embryo FET cycles; blastocyst FET cycles had the lowest risk of EP, and the differences were all statistically significant. Conclusion(s): Frozen-thawed ET cycles were associated with a statistically significantly lower risk of EP when compared with fresh cycles. These findings are consistent with ovarian stimulation being associated with an increased risk of EP. (C) 2014 by American Society for Reproductive Medicine.