Comparison of outcomes of frozen-thawed transfer of day 5 blastocysts and day 6 blastocysts

INTRODUCTION: The aim of this study was to evaluate and compare the outcomes of frozen-thawed embryo transfer of blastocysts that were expanded at the 5th day of fertilization and blastocysts expanded at the 6th day of fertilization.
METHODS: Patients who underwent frozen-thawed embryo transfer cycles between 2012 and 2020 in a single infertility center were retrospectively evaluated. A total of 1814 women with frozen embryo transfers of day 5 blastocysts and 227 women with frozen embryo transfers of day 6 blastocysts were included in the study. Outcomes of embryo transfer cycles of day 5 and day 6 blastocysts were compared.
RESULTS: Cancellation rate in day 6 blastocyst transfer group was significantly higher than day 5 transfers (5.7% vs. 0.3%; p<0.001). Post-thaw embryo quality in day 5 blastocyst group was better than day 6 blastocyst group. Clinical pregnancy rates (34% vs. 16.7%), multiple pregnancy rates (3.9% vs. 0.9%) and live birth delivery rates (31.4% vs. 15.7%) in day 5 blastocyst transfer group were significantly higher (p<0.001, p<0.001 and p<0.001). Gestational ages at birth (37.15±3.23 weeks vs. 37.97±2.52 weeks; p=0.308) and mean birthweights of infants (3153.20±681.96 g vs. 3127.72±656.75 g; p=0.994) were similar among groups. Significantly increased number of miscarriages were observed in day 5 blastocyst group (5.3% vs. 1.8%; p=0.019).
DISCUSSION AND CONCLUSION: Clinical pregnancy and live birth delivery rates of frozen-thawed transfer of day 5 blastocysts are significantly higher in comparison to transfer of their day 6 counterparts.