Abstract: Objective：To evaluate the endometrial thickness on the day of hCG injection (the hCG day), clinical pregnancy rate and embryo implantation rate in those patients undergoing the in vitro fertilization-embryo transfer (IVF-ET) with the GnRHa long protocol or GnRH antagonist (GnRHA) protocol, in order to find whether GnRHA has a negative effect on endometrial receptivity. Methods：Those cases with the GnRHa long protocol (the long-protocol group, 1 459 cases) or the GnRHA protocol (the GnRHA group, 395 cases) in our Center from January 2015 to June 2017 were enrolled. The basic characteristics of the two groups were compared. According to the age, patients were divided into four subgroups: the ＜30 years old group (352 cases), the 30-35 years old group (781 cases), the 35-40 years old group (601 cases) and the ≥40 years old group (120 cases). According to the number of eggs, patients were divided into three subgroups: the low response group (oocytes ≤3 218 cases), the normal response group (oocytes 4-14, 1 535 cases) and the high response group (oocytes ≥15 101 cases). The endometrial thickness on the hCG day, clinical pregnancy rate, embryo implantation rate and early abortion rate were compared. Results：There was no significant difference in BMI between the two groups (P＞0.05). There were significant differences between the two groups in years, infertility years, the basic level of FSH, AFC and average implanted embryos number (P＜0.05). In those cases of the ＜30 years old subgroup and the ≥40 years old subgroup, the differences in the endometrial thickness on the hCG day and embryo implantation rate between the two groups were no significant (P＞0.05), while the difference in the clinical pregnancy rate was significant in the ＜30 years old group (P＜0.05). In those cases of the 30-35 and 35-40 years old subgroups, the endometrial thickness, embryo implantation rate and clinical pregnancy rate in the GnRHa long protocol group were significantly higher than those in GnRHA protocol group (P＜0.05). In those cases of the low response subgroups, there were less transplanted embryos and higher embryo implantation rate in the GnRHa long protocol group (P＜0.05). In those cases of the normal response subgroups, the endometrial thickness on the hCG day, embryo implantation rate and the clinical pregnancy rate were higher in the GnRHa long prolocol group (P＜0.05). In those cases of the high response subgroups, there were no significant differences between the two groups. Conclusions：In those cases of the 30-40 year old and normal ovarian response population, the GnRHA protocol may reduce the endometrial receptivity, and to reduce the embryo implantation rate and clinical pregnancy rate.

Key words: Ovulation induction, Fertilization in vitro, Embryo transfer, Gonadotropin-releasing hormone