Ectopic pregnancy after in vitro fertilization: differences between fresh and frozen-thawed cycles.

To evaluate whether the uterine environment is associated with the risk of ectopic implantation by comparing outcomes of fresh and frozen-thawed embryo transfers.Retrospective historical cohort.Not applicable.We used the Society for Assisted Reproductive Technologies (SART) database to identify pregnancies that resulted from fresh and frozen blastocyst transfers from 2008 to 2011.None.We determined the proportion of ectopic (EP) versus intrauterine-only pregnancies resulting from fresh or frozen embryo transfers in autologous and donor-oocyte cycles. Generalized estimation equation regression models were used to adjust for maternal and treatment characteristics.Among 103,070 cycles that resulted in a clinical pregnancy, 1.38% were ectopic. The odds of EP were 65% lower in women who had a frozen compared with a fresh transfer in autologous cycles. Donor-oocyte transfers had lower odds of EP compared with autologous cycles, with no difference between fresh and frozen donor transfers. Women who had both a fresh and a frozen transfer with autologous oocytes had a higher risk of EP in their fresh cycles compared with their frozen cycles.Embryo transfers in cycles without ovarian hyperstimulation, such as frozen or donor cycles, were associated with lower rates of EP compared with fresh autologous cycles, suggesting that a difference in the tubal-uterine environment contributes to abnormal implantation after IVF.