Ectopic pregnancy in frozen-thawed embryo transfer: a retrospective analysis of 4,034 cycles and related factors.

We analyzed the incidence of ectopic pregnancy in frozen-thawed embryo transfer (FET) as a function of natural and hormone replacement cycles and ectopic pregnancy-related factors. In this study, there were 4,034 FET cycles performed in our center between January 2005 and December 2010, and the rates of ectopic pregnancy were compared between natural and hormone replacement cycles. The analysis of ectopic pregnancy-related factors in FET was performed with 1:4 age-matched chi-square tests. The rate of ectopic pregnancy was lower in natural FET cycles (1.46%) than in hormone replacement FET cycles (3.31%) with a statistical significance (P < 0.05). Many factors were associated with ectopic pregnancy in FET, but only treatment protocols were considered as a controllable factor. We conclude that the incidence of ectopic pregnancy is significantly lower in natural FET cycles than in hormone replacement FET cycles. The application of exogenous sex hormones in assisted reproductive cycles may be an important factor to cause ectopic pregnancy in FET. This suggests that care should be taken when selecting the treatment protocol in order to avoid ectopic pregnancy.