Abstract: Objective：To compare the pregnancy outcomes bewteen the fresh embryo transfer (ET) and the first frozen thawed embryo transfer (FET) after embryo vitrification in patients with different ovarian response on the antagonist protocol. Methods：A total of 251 cycles of infertile patients undergoing IVF/ICSI-ET and the antagonist protocol of ovarian stimulation were retrospectively studied from May 2016 to September 2018 in our Reproductive Center. According to the ovarian response, those patients were divided into three groups, the group A including 33 cycles (the number of oocytes retrieved ≤3), the group B including 157 cycles (the number of oocytes retrieved 4 to 15) and the group C including 47 cycles (the number of oocytes retrieved ＞15). Further, each group was divided into the fresh embryo transfer sub-group (A1 of 18 cycles, B1 of 99 cycles, C1 of 15 cycles) and the first frozen thawed embryo transfer after embryo vitrification sub-group (A2 of 15 cycles, B2 of 58 cyles, C2 of 46 cycles). Basic parameters, the embryo implantation rate and the clinical pregnancy rate were compared. Results：There were no significant differences in age, body mass index (BMI), bFSH/bLH, sterility type, total Gn, days of Gn, insemination mode and insemination rate in A1/A2 group, B1/B2 group and C1/C2 group (all P＞0.05). The number of oocytes retrieved in the C2 sub-group was higher than that in C1 sub-group (t=-2.963, P=0.000). As for the outcomes, the clinical pregnancy rate in B2 sub-group was significantly higher than that in B1 sub-group ( χ2=5.502, P=0.019). The embryo number and embryo quality score in C2 sub-group were significantly higher than those in C1 sub-group (P＜0.05). However, there were no significant differences in the implantation rate, clinical pregnancy rate and abortion rate (all P＞0.05). Conclusions：Patients with low ovarian response should be recommended the fresh transplantation as priority selection. Patients with normal ovarian response should give priority to the whole-embryo freezing and then transfer the frozen-thawed embryo. In order to avoid the ovarian hyperstimulation to the greatest extent, patients with ovarian hyperresponsiveness should be directly recommended to give priority to the whole-embryo freezing and then transfer the frozen-thawed embryo.

Key words: Receptors, gonadotropin, Ovulation induction, Reproductive techniques, assisted, Fresh embryo transfer, Frozen thawed embryo transfer