Abstract: Objective：To compare the pregnant outcomes of single-blastocyst transfer in those high ovarian responders with the different levels of serum estradiol on the day of human chorionic gonadotrophin (hCG) injection in their ovarian stimulation cycles. Methods：A total of 1 167 cycles in those high ovarian responders who performed embyo transfer in their fresh stimulation cycles were retrospectively analyzed. According to the concentration of serum estradiol (E2) on the day of hCG administration, 1 167 cycles were divided into four groups as follows. The group A (n=59) was the D5 single-blastocyst transfer group while E2＜5 000 pg/mL. The group B (n=219) was the D3 embyo transfer while E2＜5 000 pg/mL. The group C (n=387) was D5 single-blastocyst transfer while E2≥5 000 pg/mL. The group D (n=502) was the D3 embyo transfer while E2≥5 000 pg/mL. The basical characteristics and pregnancy outcomes were compared between the group A and B, and between the group C and D. Results：The number of retrieved oocytes，mature oocytes/MⅡ oocytes, 2PN fertility rate, number of embyo transfered in the group A were higher than those in the group B, while the twins pregnancy rate and twins delivery rate were lower than those in the group B (all P＜0.05). However, there were no significant differences in the implantation rate, clinical pregnancy rate, abortion rate, live birth rate and moderate-severe ovarian hyperstimulation syndrome (OHSS) between the group A and the group B (P＞0.05). The bFSH and the dose of gonadotroph in the group C were lower than the group D, while the AFC, the number of retrieved oocytes, mature oocytes/MⅡ oocytes, the concentrations of serum E2, progestron (P) on the day of hCG, 2PN fertility rate, 2PN cleavage rate, number of embyo transfered, number of high quality in the group C were higher than those in the group D (all P＜0.01). Interestingly, the implantation rate in the group C was higher than group D, while the twins pregnancy rate, twins delivery rate and the rate of moderate-severe OHSS were significantly lower than those in the group D (all P＜0.05). There were no significant differences in the clinical pregnancy rate, abortion rate, live birth rate between the group C and the group D (P＞0.05). Conclusions：Compared with the two cleavage embryos transfer，the single-blastocyst transfer under the different concentrations of E2 on the day of hCG stimulation can ensure the similar pregnancy rate and live bith rate, while significantly reduce the twins pregnancy rate and twins delivery rate. Meanwhile, the incidence of OHSS is not increased in those cycles with the higher level of estradiol.
 

Key words: Fertilization in vitro, Ovarian hyperstimulation syndrome, Estradiol, Single-blastocyst transfer