Does Developmental Timing Matter? Comparative Analysis of Day 5 and Day 6 Euploid Blastocyst Transfers in Recurrent Implantation Failure Patients

<b>Background/Objectives</b>: The timing of blastocyst formation is an important factor in in vitro fertilization (IVF) outcomes. While many studies have shown similar pregnancy rates for euploid blastocyst transfers occurring on day 5 or 6, controversy remains, especially regarding patients with recurrent implantation failure (RIF). This study aimed to evaluate whether day 5 (D5) and day 6 (D6) euploid blastocysts differ in terms of the clinical outcomes achieved after frozen-thawed euploid embryo transfer, with subgroup analysis by maternal age in RIF patients. <b>Methods</b>: This retrospective analysis included a cohort of 514 patients who experienced RIF, categorized into two distinct groups: the initial group consisting of individuals who underwent embryo biopsy on day 5 (<i>n</i> = 456 euploid transfers) and the subsequent group comprising patients who underwent biopsy on day 6 (<i>n</i> = 58 euploid transfers) following ovum retrieval. These groups were utilized based on the completion of blastocyst development and the eventual pregnancy outcomes after the first post-frozen-thawed euploid embryo transfer confirmed by PGT-A. <b>Results</b>: Out of the total cohort of 514 RIF patients, 472 individuals were younger than 40 years of age, representing 91.8% of the sample population. The IVF process yielded successful positive pregnancy outcomes in 85.4% of the cases (439 out of 514)., and there was no statistically significant difference in maternal ages between the day 5 and day 6 biopsy cohorts (<i>p</i> = 0.286). The proportion of successful clinical pregnancies per transfer was markedly higher among those who underwent biopsy on day 5 in comparison to those subjected to biopsy on day 6 (79.2% vs. 58.6%, <i>p</i> = 0.002). In patients aged 35 years and younger who underwent biopsy on day 5, the rates of successful clinical pregnancy outcomes were superior to those who underwent biopsy on day 6 (79.8% vs. 56.5% <i>p</i> = 0.016). In individuals older than 35 years of age, no significant discrepancies in clinical pregnancy rates were observed between the two groups. <b>Conclusions</b>: Day 6 euploid embryo transfer was associated with lower clinical pregnancy rates per transfer compared with those on day 5, particularly in patients younger than 35 years of age. In women ≥ 35 years of age, outcomes did not differ significantly. These outcomes suggest that developmental timing interacts with maternal age in determining clinical outcomes. Further prospective studies with larger and more balanced cohorts are needed to confirm these findings.