L26/P-123 When morphology is not enough: interaction of blastulation timing and morphology in euploid embryo transfer outcomes

Abstract Study question In elective single euploid blastocyst transfers, does delayed blastulation modify the prognostic value of morphology for pregnancy outcomes? Summary answer Delayed blastulation attenuates the prognostic value of morphology, with some Day-6 euploid embryos showing outcomes comparable to lower-grade Day-5 embryos. What is known already Although euploid embryos are recognized to differ in implantation potential, guidance on embryo prioritization when morphology and blastulation timing conflict remains limited. Blastocyst morphology and day of development independently predict outcomes in unselected embryos, yet their interaction within euploid-only cohorts is less clearly defined. Existing studies often assess these parameters in isolation and provide limited support for clinical decision-making when delayed blastulation accompanies favorable morphology. Study design, size, duration Retrospective observational study (2014–2024) including 613 elective single euploid frozen embryo transfers. Only self-gamete IVF/ICSI cycles with blastocyst biopsy, NGS-based PGT-A, and euploid transfer were included; donor cycles, non-euploid transfers, and fresh transfers were excluded. Transfers were stratified by day of blastulation (Day-5: n = 560; Day-6: n = 53) and subdivided by Gardner morphology grades. Participants/materials, setting, methods Blastocysts expanding on Day-5 or Day-6 underwent trophectoderm biopsy followed by vitrification using a freeze-all strategy. Genetic screening was performed using NGS, and only euploid embryos were transferred in frozen embryo transfer cycles. Elective single euploid transfers with 100% post-warming survival were included. Implantation, miscarriage, and live birth outcomes were compared across morphology grades and blastulation timing. Main results and the role of chance Main results and the role of chance (≤200 words)Among 560 Day-5 euploid blastocyst transfers, implantation and live birth rates declined progressively with worsening morphology, from approximately 75% in AA-grade embryos to 50% in CC-grade embryos, demonstrating a significant dose–response relationship (p for trend <0.05). Miscarriage rates increased with declining morphology, although not all comparisons reached statistical significance.In the Day-6 cohort (n = 53), pregnancy outcomes were consistently lower across morphology grades compared with Day-5 transfers. Within-grade comparisons showed significantly reduced live birth rates for Day-6 BC-grade embryos (n = 20) compared with Day-5 BC embryos (p = 0.013), while CC-grade embryos (Day-6 n = 24) showed a borderline reduction (p = 0.051). Across comparable grades, Day-6 blastocysts exhibited an absolute reduction of approximately 20–30% in live birth rates, driven by lower implantation and higher miscarriage rates. Notably, high-grade Day-6 blastocysts did not consistently outperform lower-grade Day-5 embryos, highlighting a clinically relevant trade-off frequently encountered in practice. Analyses were stratified by blastulation day to account for unequal group sizes reflecting routine clinical distribution. Limitations, reasons for caution This retrospective single-center analysis is limited by imbalance between Day-5 and Day-6 sample sizes and small Day-6 subgroups, reducing statistical power. Findings should be interpreted as hypothesis-refining rather than practice-changing. Wider implications of the findings These data do not propose a new selection algorithm but provide confirmatory evidence that blastulation timing should be considered alongside morphology when prioritizing euploid embryos. The findings may support counseling and transfer decisions when morphology and developmental timing present competing signals. Trial registration number No