The influence of blastocyst quality versus blastulation speed on pregnancy outcomes in vitrified-warmed single embryo transfer cycles: Insights from pre-implantation genetic testing for aneuploidy

Objective: To evaluate the impact of blastulation speed and blastocyst quality on pregnancy outcomes in single vitrified-warmed blastocyst transfer (SVBT) cycles. Methods: A total of 736 SVBT cycles conducted between 2019 and 2021 were analyzed. Blastocysts were classified according to blastulation speed (day 5 vs. day 6) and morphological quality (top, good, average, and poor). A supplementary analysis included 395 blastocysts from 171 cycles that underwent pre-implantation genetic testing for aneuploidy between 2022 and 2023. Ongoing pregnancy rates and euploidy rates were compared across blastulation speed and quality categories. Results: In SVBT cycles, the ongoing pregnancy rate was significantly higher in the day 5 group than in the day 6 group. However, no significant difference in ongoing pregnancy rates was observed between top-quality and good-quality blastocysts prior to vitrification, irrespective of blastulation speed. Average- and poor-quality blastocysts were more frequently observed in the day 6 group than in the day 5 group. In addition, euploidy rates were significantly higher in the day 5 group than in the day 6 group. However, no significant difference in euploidy rates was detected between top- or good-grade blastocysts derived from day 5 and day 6 embryos. By contrast, significant differences in euploidy rates were observed between average- and poor-quality blastocysts from day 5 and day 6. Conclusion: Blastocyst quality has a significant influence on clinical outcomes, whereas blastulation speed appears to be comparatively less critical. For slow-developing embryos, vitrification of high-quality day 6 blastocysts may be preferable to the transfer of low-grade day 5 blastocysts, thereby providing practical clinical options for patients experiencing developmental delays.