Abstract
OBJECTIVE: The aim of this study is to assess the impact of different embryo transfer strategies, focusing on cleavage-stage embryos and blastocysts, on pregnancy and neonatal outcomes in frozen-thawed embryo transfer (FET) cycles among women < 35 years old and ≥ 35 years old. METHODS: A retrospective cohort analysis of 3,065 FET cycles performed between April 2015 and October 2022 categorized patients into seven groups by embryo morphology, quality, and quantity: single/double high-quality cleavage (A/B), single/double high-quality blastocyst (C/D), single/double poor-quality blastocyst (E/F), and mixed-quality blastocyst (G). Stratified by age (<35/≥35 years), outcomes (clinical pregnancy, live birth, multiple pregnancy, preterm birth) were analyzed using non-parametric tests and chi-square. RESULTS: In women <35 years, transferring two high-quality cleavage embryos (Group B) yielded higher clinical pregnancy (60.12% vs 28.57%) and live birth rates (51.45% vs 19.64%) than single high-quality cleavage embryos (Group A; P < 0.05). For blastocyst transfer, double high-quality blastocysts (Group D) showed higher multiple pregnancy and preterm birth rates than single high-quality blastocysts (Group C; P < 0.05). In women ≥35 years, Group B had superior pregnancy outcomes compared to Group A, while Group D demonstrated significantly higher clinical pregnancy and live birth rates than Group C (P < 0.05). Transferring two poor-quality blastocysts (Group F) improved live birth rates in older patients compared to single poor-quality blastocysts (Group E; 31.91% vs 14.29%, P < 0.05). No significant differences in neonatal birth weight were observed across age groups. CONCLUSION: For FET cycles, transferring two high-quality cleavage embryos is recommended for all ages. Women <35 years should prioritize single high-quality blastocyst transfer to minimize multiple pregnancies and preterm births, while those ≥35 years benefit from double high-quality blastocysts. Transferring two poor-quality blastocysts may enhance pregnancy outcomes in older patients. These findings warrant validation through multicenter studies to ensure clinical applicability.