Live birth rate following frozen-thawed blastocyst transfer is higher in high-grade Day 6 blastocyst than low-grade Day 5 blastocyst

Abstract Background Day 5 (D5) blastocyst is generally given priority to transfer than Day 6 (D6) blastocyst, however, which one should be prioritized to transfer when only low-grade D5 and high-grade D6 blastocysts are available? Methods A large retrospective cohort study was carried out to evaluate the live birth rate (LBR) following D5 blastocyst and excellent D6 blastocyst in single frozen-thawed blastocyst transfer (FBT) during Jan 2014 and Dec 2018. The biopsied blastocysts form consecutive PGT-A case series during Feb 2013 to Dec 2021 were performed as a supplementary analysis. Results The LBR was highest in high-grade D5 blastocyst (57.60%) and lowest in low-grade D6 blastocyst (29.72%, vs high-grade D5, aOR 0.38, 95%CI 0.30–0.48, p < 0.001). The LBR achieved in high-grade D6 blastocyst transfer was significantly higher than low-grade D5 blastocyst (50.43% vs 40.70%, aOR 1.54, 95%CI 1.05–2.26, p = 0.027), and is comparable with that in high-grade D5 blastocyst (50.43% vs 57.60%, aOR 0.89, 95%CI 0.61–1.32, p = 0.568). There were no significant differences in preterm birth rate, very preterm birth rate, mean live birth weight, birth weight < 1500g and > 4000g between four cohorts. As for aneuploidy analysis in PGT, there were 54.55% of euploid blastocysts (30/55) among high-grade D6 blastocysts, significantly higher than 41.39% of euploid blastocysts (565/1365) among low-grade D5 blastocyst (p < 0.001). Conclusions Our data suggested Day 6 blastocyst with high morphology grading should be preferred than Day 5 blastocyst with low morphology grading when selecting blastocyst transfer to short the time of conceiving. Trial registration: This study was approved by the hospital institutional ethics committee (No. 2021002).