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.