Abstract: Objective　To observe the effect of progesterone (P) level on the pregnancy outcome of natural cycle frozen thawed embryo (FET). Methods　A retrospective analysis was performed on 549 FET cycles with natural cycles, including 211 cycles in the cleavage embryo group and 338 cycles in the blastocyst group. The two groups were divided into 4 subgroups according to P level on transplantation day. The divided embryo groups were: the ≤10 μg/L group (n=15), the＞10-20 μg/L group (n=84), the ＞20-30 μg/L group (n=70) and the ＞30 μg/L group (n=42). The divided blastocyst groups were: the ≤15 μg/L group (n=35), the ＞15-25 μg/L group (n=135), the ＞25-35 μg/L group (n=83) and the＞35 μg/L group (n=85). Data of age, endometrial thickness, average number of transplanted embryos, diameter of follicle before ovulation, hormone level on the day of transplantation, number of transplanted embryos, clinical pregnancy rate, implantation rate, abortion rate, ectopic pregnancy rate, delivery rate and male/female ratio were compared between the blastocyst group, the split embryo group and the subgroups. Results　The average number of embryo transfer, ectopic pregnancy rate were significantly higher in the splitting embryo group than those of the blastocyst groups. Data of E2, P level, childbirth, implantation rate and pregnancy rate were higher in the blastocyst transplant group than those of the splitting embryo group (P＜0.01). There were no significant differences in age, lining thickness, diameter of follicle before ovulation, abortion rate and ratio of male to female births between the two groups. There were no significant differences in age, endometrial thickness, average number of transplanted embryos, follicular diameter and E2 level, implantation rate, pregnancy rate, abortion rate, ectopic pregnancy rate and delivery rate between the subgroups of split embryo group and the blastocyst group. Conclusion　The implantation rate of blastocyst is significantly higher than that of split embryo in natural periodic FET. Although P level on transplantation day shows no significant effect on pregnancy outcome in blastocyst or split embryo, it should be more cautious when selecting transplanting split embryo.

Key words: progesterone, embryo transfer, blastula, pregnancy outcome, freezing, pregnancy ratel, endometrium, cleavage embryo, natural cycle