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.