Objective To compare the pregnancy and neonatal outcomes of blastocysts cultured to Day 6（D6）or Day 7（D7）with those cultured to Day 5（D5）in frozen-thawed embryo transfer（FET）cycles，and to investigate whether extended blastocyst culture increases the risk of adverse obstetric and neonatal outcomes.Methods This single-center retrospective cohort study included 2，325 patients who underwent single frozen-thawed blastocyst transfer at the Reproductive Center of Peking University Third Hospital between January 2010 and December 2023.Among them，465 cycles were D7，930 cycles were D6，and 930 cycles were D5.The three groups were matched at a 1∶2∶2 ratio based on maternal and paternal age，female body mass index（BMI）cause of infertility，and fertilization method.Clinical characteristics，pregnancy outcomes，obstetric complications，and neonatal outcomes were compared between the D6/D7 groups and the D5 group.Results The proportion of primary infertility was significantly higher in D7 group compared to D5 group（65.8% vs. 57.5%，P<0.05）.D6 and D7 groups showed significantly lower proportions of top morphological grade embryos（63.5% vs. 77.3%，P<0.001；38.5% vs. 77.3%，P<0.001）.D7 group had significantly lower biochemical pregnancy rates（32.5% vs. 47.2%，P<0.001），clinical pregnancy rates（26.0% vs. 42.2%，P<0.001），and live birth rates（17.6% vs. 32.2%，P<0.001）compared to D5 group.However，no significant differences were found in multiple pregnancy rates（D6：1.1%，D5：1.8%，D7：2.5%），and miscarriage rates（D6：22.5%，D5：20.9%，D7：28.9%）between D5 and D6/D7 groups.While D6 group had a significant difference in pregnancy complications（D6：6% vs. D5：11.2%，P=0.011），no significant differences were observed between D5 and D7 groups（D5：11.2% vs. D7：13.2%）.The terminations of mid-trimester and late pregnancy with fetal malformation included three cases，a Down syndrome and a fetal developmental malformation in D6 group，and a bilateral renal agenesis in D7 group.Compared to D5 group，D6 or D7 group was not different in preterm birth rates（D6：8.8%，D5：10.7%，D7：12.2%），congenital birth defects rates（D6：1.1%，D5：1.0%，D7：2.4%），average birth weight[D6：3400（3060-3700），D5：3435（3100-3750），D7：3400（3100-3690）]，and average gestational age at birth[D6：39（38-39），D5： 39（38-39），D7：39（38-39）].Conclusion In frozen-thawed embryo transfer cycles，transferring Day 6 or Day 7 blastocysts does not significantly increase the risk of pregnancy-related complications or adverse neonatal outcomes compared to Day 5 blastocysts.