Delayed blastulation and pregnancy outcomes in single frozen-thawed euploid blastocyst transfer

Objective This study aimed to evaluate the impact of embryo developmental speed on clinical outcomes in euploid blastocyst transfers, with a focus on maternal age and embryo morphological quality. Methods A retrospective cohort study was conducted, including patients who underwent single blastocyst transfer following preimplantation genetic testing. Embryos were categorized based on developmental speed (day 5 vs . day 6 blastocysts) and further stratified by maternal age and embryo morphological quality. Clinical outcomes were compared between groups. Results Day 5 euploid blastocysts yielded significantly higher implantation (68.8% vs . 48.3%) and live birth rates (63.7% vs . 40.4%) than day 6 blastocysts. In the advanced-age group, day 5 euploid embryos demonstrated significantly higher implantation (72.5% vs . 40.5%) and live birth rates (68.8% vs . 28.6%) than day 6 embryos. Developmental speed influenced pregnancy outcomes in low-quality embryos, where day 6 blastocysts exhibited a reduced live birth rate (29.3% vs . 65.5%) than day 5 blastocysts. Conclusion Our findings indicate that day 5 euploid embryos are associated with better pregnancy outcomes compared to day 6 euploid embryos, with this trend being more pronounced in older participants and lower-quality embryo groups. However, day 6 euploid blastocyst transfer also yields acceptable implantation and pregnancy outcomes.