Does blastocyst transfer have a higher live birth rate than cleavage-stage embryo transfer in patients with delayed in vitro blastocyst development?

When embryo cohorts exhibit delayed blastocyst development, with only slow-developing day 6 (D6) blastocysts available for transfer, does blastocyst transfer achieve a higher live birth rate (LBR) than cleavage-stage embryo transfer?

A modified blastocyst culture strategy was applied in freeze-all cycles. Specifically, one or two good-quality embryos were cryopreserved on day 3 (D3) as a backup, while an equal or greater number of good-quality embryos were cultured to the blastocyst stage. Among freeze-all patients managed with this strategy, 275 patients were identified who had only D6 blastocysts available for transfer, with no transferable day 5 (D5) blastocysts following extended in vitro culture. Cumulative LBR after one oocyte retrieval over 2 years and pregnancy outcomes in the first frozen-thawed embryo transfer (FET) cycles were compared between groups with priority transfer of backup D3 cleavage-stage embryos versus top-graded D6 blastocysts.

The cumulative LBR was not significantly different between the group with priority transfer of backup cryopreserved D3 embryos and the group with priority transfer of top-quality D6 blastocysts [50.6% vs. 48.4%; adjusted odds ratio (aOR), 0.759; 95% CI, 0.384-1.499; p = 0.427]. In the first FET cycles, transfer of top-graded D6 blastocysts did not yield a higher LBR compared with transfer of backup cryopreserved D3 embryos [36.3% vs. 43.5%; aOR, 0.601; 95% CI, 0.299-1.208; p = 0.153]. Subgroup analyses showed no significant differences in LBR between the two groups following either single embryo/blastocyst transfer (SET/SBT) (37.0% vs. 36.4%; aOR, 0.922; 95% CI, 0.234-3.634; p = 0.907) or double embryo/blastocyst transfer (DET/DBT) (35.4% vs. 44.6%; aOR, 0.493; 95% CI, 0.213-1.138; p = 0.097). No statistically significant differences were observed in clinical pregnancy rate, obstetric complications, or neonatal outcomes between the two groups in the first FET cycles.

When embryo cohorts exhibit delayed development at the blastocyst stage, blastocyst transfer does not result in a higher LBR and therefore may not confer an advantage in achieving live birth sooner compared with the transfer of good-quality cleavage-stage embryos.