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

Abstract

Objective: The aim of this study was to evaluate and compare the outcomes of frozenthawed embryo transfer of blastocysts that were expanded at the 5th day of fertilization and blastocysts expanded at the 6th day of fertilization. Material and 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). 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.