Objective  To investigate the effect of the endometrial thickness and embryo transplantation (ET) time on the incidence of ectopic pregnancy in frozen thawed embryo transplantation.

  Methods  A prospective study of 11 681 frozen-thawed embryo transplantation cycles in the Reproductive Center of Northwest Women and Children′s Hospital was conducted to analyze the pregnancy outcomes, including 6 901 cycles of clinical pregnancy and 75 of ectopic pregnancy. According to the embryo transplantation time, the FET cycles were divided into D₃ embryo transplantation group (5 912 cycles in embryo period) and D₅ blastula transplantation group (5 949 cycles in blastula period). And were further subdivided into ≤9 mm group, >9~12 mm group and >12 mm group according to endometrial thickness. The incidence of ectopic pregnancy of varied groups were compared.

  Results  There were 45 cases with ectopic pregnancy in the D₃ embryo transplantation group, and 30 cases in D₅ blastocyst transplantation group, and its incidence of ectopic pregnancy showed a significant difference(P < 0.05). Compared with ≤9 mm group, the incidence of ectopic pregnancy in >9~12 mm group and >12 mm group were significantly lower.

  Conclusion  D₅ blastula transplantation group has significantly lower risk of ectopic pregnancy than D₃ embryo transplantation group, endometrial thickness can be independent factor of ectopic pregnancy.