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

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).

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.

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.

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.