Pregnancy Outcomes for Single Frozen-thawed Blastocyst Transfer of Day 5 and Day 6 With Different Quality: a Large Matched-cohort Study

Abstract Background: Whether the day of blastocyst expansion affects pregnancy outcomes in frozen-thawed blastocyst transfer (FBT) cycles remains controversial. The conflicting results may be attributed to the potential confounders of FBT cycles between day 5 and day 6. The aim of this study is to compare the clinical pregnancy rate (CPR) and live birth rate (LBR) of FBT cycles on day 5 versus day 6 accounting for the baseline comparability and determine the transfer order of FBT cycles. Methods: This was a retrospective match-cohort study, FBT cycles of day 5 and day 6 were matched by the general characteristics of patients, 2207 matched pairs of FBT cycles were included from January 2016 to December 2019 in our reproductive medicine center. Results: The CPR and LBR of day 5 blastocyst were significantly higher than day 6 with the same quality. In the FBT cycles with good quality, the CPR of day 5 and day 6 was 61.30%, 57.56% respectively (P=0.045), the LBR was 44.79%, 36.16% respectively (P<0.001). While in the FBT cycles with poor quality, the CPR of day 5 and day 6 was 48.61%, 40.89% respectively (P=0.006), the LBR was 31.71%, 25.74% respectively (P=0.019). The CPR of FBT cycles on day 6 with good quality was statistically higher than day 5 with poor quality (57.56% versus 48.61%, P=0.001). Maternal age, AMH, endometrial thickness, embryo quality and the day of blastocyst expansion was independently correlated with CPR and LBR. The FBT cycles of day 5 exhibited significantly higher CPR [adjusted odds ratio (OR)=1.246, 95% confidence intervals (Cl): 1.097-1.415, P=0.001] and LBR (adjusted OR=1.435, 95% Cl: 1.258-1.637, P<0.001) compared with day 6. Conclusions: The embryo quality should be the primary indicator in the FBT cycles. The blastocyst of day 5 should be preferred if the embryo quality of day 5 and day 6 is equal. Trial registration: Not applicable.