Abstract: Objective：To investigate the optimum blastocyst culture time after thawing of vitrificated cleaved embryos and its effect on pregnant outcome. Methods：477 patients conducting 520 cycles of blastocyst culture after thawing of vitrificated cleaved embryos were retrospectively analyzed. According to the blastocyst culture time after thawing, 520 cycles were grouped into two groups: Group D2 (the warming time: 15-16 o′clock in D2) and Group D3 (the warming time: 8-9 o′clock in D3). In both groups, blastocysts were observed on 8-9 o′clock in D5/D6. Therefore the culture times were 65 h in Group D2 and 48 h in Group D3 in D5, and 89 h and 72 h in D6. Results：There were no significant differences between Group D2 and Group D3 in the blastocyst formation rate (48.10% vs. 46.59%), good quality blastocyst formation rate (39.70% vs. 34.22%), clinical pregnancy rate (70.08% vs. 69.48%), biochemical pregnancy rate (7.48% vs. 7.98%) and implantation rate (55.93% vs. 58.06%) (P＞0.05). The blastocyst formation rates of good and non-good embryos were (30.37% vs. 13.28%) and (62.64% vs. 40.14%) in 48 h and 65 h, respectively (P＜0.05). However, the blastocyst formation rates of good and non-good embryos were (32.32% vs. 27.59%) and (2.17% vs. 2.06%) in 72 h and 89 h respectively (P＞0.05). The clinical pregnancy rates and implantation rates were showed: 48 h group＞65 h group＞72 h group＞89 h group, and the biochemical pregnancy rates were showed: 89 h group＞48 h group＞65 h group＞72 h group, although there were non-significant differences in those rates among groups (P＞0.05). Conclusions：We can warm the vitrificated cleaved embryos at 15-16 o′clock in D2 or at 8-9 o′clock in D3. Majority of formed blastocysts can be transplanted into uterus in D5, if the vitrificated cleaved embryos were warmed at 15-16 o′clock in D2. The blastocysts of good quality embryos can be formed in D5 or D6, however those non-quality embryos could spend more time to form blastocysts until D6.

Key words:  Embryo transfer, Blastula, Embryo culture techniques, Frozen-thawed embryo transfer