Comparative Evaluation of Clinical Outcomes and Aneuploidy Risk Between Day 5 and Day 6 Blastocyst: A Retrospective Study

AIM: Embryo selection in assisted reproduction is pivotal for optimizing pregnancy outcomes. While Day 5 blastocysts are generally preferred due to earlier development, the clinical efficacy of high-quality Day 6 embryos remains debatable, particularly concerning aneuploidy risk and pregnancy loss. This study aims to compare chromosomal euploidy rates and clinical outcomes between Day 5 blastocysts and Day 6 blastocysts after preimplantation genetic testing for aneuploidy (PGT-A). METHODS: This retrospective cohort study included 349 frozen embryo transfer cycles (Day 5: n = 165, Day 6: n = 184). All embryos underwent PGT-A using next-generation sequencing. Clinical outcomes, fetal loss, and ploidy status were assessed. Logistic regression identified predictors of fetal loss and aneuploidy. RESULTS: No significant difference in overall aneuploidy rates was observed between Day 5 and Day 6 embryos (47.9% vs. 48.9%, p = 0.847). However, early fetal loss (< 12 weeks) was higher among Day 6 embryos (26.6% vs. 17.6%, p = 0.043). Regression analysis revealed Day 6 transfer was associated with increased odds of fetal loss (OR = 1.707, p = 0.045), while greater blastocyst expansion and secondary infertility were protective. Higher FSH levels and inner cell mass (ICM) Grade B were significant predictors of aneuploidy. CONCLUSION: Top-quality Day 6 embryos demonstrate similar aneuploidy rates to Day 5 embryos but are associated with higher early pregnancy loss. Morphological grading and hormonal markers remain essential adjuncts in embryo selection post-PGT-A.