Pregnancy and perinatal outcomes after day 5 versus day 6 blastocyst‐stage embryo transfer: A systematic review and meta‐analysis

Abstract Introduction Embryo transfer at the blastocyst stage has become popular in assisted reproductive technology, but it is still a challenge for embryologists to select the embryos with the highest implantation potential. We aimed to investigate whether blastocyst development speed and blastocyst quality affect pregnancy and perinatal outcomes in single frozen–thawed blastocyst transfer (SFBT) cycles. Material and Methods We therefore conducted a meta‐analysis to compare the pregnancy and perinatal outcomes after day 5 versus day 6 SFBT. Pregnancy and perinatal outcomes of SFBT included HCG positive rate, clinical pregnancy rate (CPR), live birth rate (LBR), miscarriage rate (MR), multiple pregnancy rate (MPR), ongoing pregnancy rate (OPR), ectopic pregnancy rate (EPR), mean birth weight, low birth weight (LBW) rate, rate of macrosomia, mean gestational age at birth, preterm birth rate, birth defects rate, and sex ratio (female/male). Results A total of 106 316 participants were included in this meta‐analysis and were assigned to the D5 ( n = 66 329) and D6 ( n = 39 987) SFBT groups. Pooled analysis showed that the D5 SFBT group had a higher HCG positive rate (RR 1.24, 95% CI 1.14–1.34), CPR (RR 1.28, 95% CI 1.22–1.34), OPR (RR 1.36, 95% CI 1.21–1.53), LBR (RR 1.39, 95% CI 1.32–1.47), and a lower MR (RR 0.78, 95% CI 0.73–0.83) than the D6 SFBT group. No significant difference was observed between the D5 and D6 SFBT groups in other pregnancy and perinatal outcomes. Subgroup analysis comparing poor‐quality D5 blastocysts with high‐quality D6 blastocysts revealed no significant differences in most outcomes, except for a lower HCG positive rate (RR 0.92, 95% CI 0.87–0.97) and a higher rate of macrosomia (RR 2.13, 95% CI 1.05–4.31) in the poor‐quality D5 group. Conclusions The findings suggested that transfer D5 blastocysts should be prioritized over D6 blastocysts in SFBT in clinical practice. Besides, poor‐quality D5 blastocysts and D6 high‐quality blastocysts exhibited comparable outcomes. Given the overall low quality of available evidence, the association between the pregnancy outcomes and blastocyst development speed requires further investigation.