Abstract Objective To evaluate the effects of number and quality of transferred blastocysts and female age on pregnancy outcomes. Design A retrospective study. Setting Two IVF centers. Population Women undergoing first blastocyst transfer in frozen-thawed cycles. Methods 6096 frozen-thawed blastocyst transfer cycles were divided into five groups based on blastocyst number and quality and three groups according to female age. Main Outcome Measures Implantation rate (IR), clinical pregnancy rate (CPR), multiple pregnancy rate (MPR), live birth rate (LBR) and neonatal characteristics. Results Female age, quality and number of transferred blastocysts had significant effects on pregnancy outcomes. IR, CPR and LBR of the same blastocyst transfer groups decreased gradually with age. Increasing the transferred number of same grade blastocysts did not obviously increase CPR and LBR, however, it significantly increased MPR. For double blastocyst transfer, the better-quality the transferred blastocysts were, the higher the IR, CPR, MPR and LBR were. Singleton group had the best neonatal characteristics followed by dizygotic twins and monozygotic twins. There was a positive correlation between sex ratio and the proportion of good-quality blastocysts, and the difference was significant between good-quality and poor-quality blastocyst groups (1.35 vs 0.98, P/CI = 0.002/1.123-1.687). Conclusions Selective single good-quality blastocyst transfer may serve as an effective embryo transfer strategy to reduce MPR while maintaining LBR and obtain good pregnancy outcomes, but, the strategy may result in an unbalanced sex ratio of newborns. Funding None. Keywords: Blastocyst transfer, age, multiple pregnancy, live birth rate, neonatal characteristics.

Objective To evaluate the effects of different post-thawed culture periods on the clinical outcomes Design A retrospective study. Setting Two IVF centers. Population Women undergoing first cleavage-stage embryo transfer in frozen-thawed cycles. Methods 9832 FET cycles were divided into three groups according to female age: < 35, 35-39 and > 39 years, and two groups depending on post-thawed culture period: short and long culture groups. The long culture group divided into three groups depending on blastomere growth number: ≤ 2, one ≤ 2 and the other > 2, and > 2 groups. Main Outcome Measures Implantation rate (IR), clinical pregnancy rate (CPR), multiple pregnancy rate (MPR), live birth rate (LBR) and neonatal characteristics. Results Long post-thawed culture caused a significant increase in the IR, CPR, MPR and LBR (P/CI = 0.002/1.034-1.162, 0.027/1.011-1.194, 0.028/1.014-1.255 and 0.001/1.054-1.245 respectively), and blastomere growth number had a significant effect on IR, CPR, MPR and LBR (P = 0.000, 0.000, 0.000 and 0.000 respectively). No significant differences were present in neonatal characteristics between the two post-thawed culture groups. Singleton group had a higher average gestational age and birthweight as well as a lower cesarean section rate, preterm labor rate and low birthweight rate. Conclusions Long post-thawed culture was associated with higher IR, CPR, MPR and LBR, and transferring a well-developed embryo after long post-thawed culture might be a viable embryo transfer strategy to decrease MPR while maintaining CPR and LBR. Funding None Keywords Post-thawed culture, blastomere growth, neonatal characteristics, live birth rate.