The pregnancy outcomes of day-5 poor-quality and day-6 high-quality blastocysts in single blastocyst transfer cycles

Objective: This study compared the outcomes of single blastocyst transfer cycles, using day-5 poor-quality blastocysts and day-6 high -qual-ity blastocysts.Methods: We analyzed 462 frozen-thawed embryo transfer (FET) cycles performed at our center from January 2014 to December 2019. The cycles were divided into two groups: a day-5 poor-quality blastocyst transfer group (group A) and a day-6 high-quality blastocyst transfer group (group B). The clinical outcomes were tested.Results: In groups A and B, respectively, the clinical pregnancy rate (CPR; 61.65% vs. 67.17%, p=0.258), implantation rate (IR; 61.65% vs. 67.17%, p=0.258), and live birth rate (LBR; 69.51% vs. 77.83%, p=0.134) showed no significant differences. Moreover, when day-3 embryo quality was considered, the CPR, IR, and LBR were also similar in group A and group B (p>0.05).Conclusion: The clinical outcomes of day-5 poor-quality blastocysts and day-6 high-quality blastocysts were similar, suggesting that the de-velopmental speed of the embryo might be more important than embryo quality for the clinical outcomes of single blastocyst transfer in FET cycles.