Euploid Day-5 Blastocysts Versus Euploid Day-6 Blastocysts — Will the Reproductive Outcomes Differ? An Observational Study

Background and objective: Day-5 blastocyst embryos are usually chosen for assisted reproductive therapy. We compared the reproductive outcomes of the euploid blastocysts developed on Day 5 versus Day 6. Methods: This single-center, retrospective observational study analyzed patients aged 25–45 years, who underwent intracytoplasmic sperm injection from December 2014 to November 2018. Depending on the day of trophectoderm biopsy, patients were categorized into Day-5 and Day-6 groups. Percentages of euploid embryos were calculated for both groups, and elective single euploid blastocysts were transferred in a frozen embryo transfer (FET) cycles. The study endpoints were the comparisons of the reproductive outcomes including clinical pregnancy rate (CPR), implantation rate (IR), miscarriage rate (MR), and live birth rate (LBR) between Day-5 and Day-6 euploid FET groups. Results: A total of 801 embryos from 184 patients were evaluated [Day 5 ([Formula: see text]=769); Day 6 ([Formula: see text]=32); 42.45% were euploid] with the rate of euploidy in Day-5 and Day-6 groups at 42.52% and 40.62%, respectively. A total of 126 patients underwent FET with 126 elective single euploid embryos (Day 5: 117; Day 6: 9). For Day-5 versus Day-6 groups, a significantly higher IR (61.54% vs. 44.44%; [Formula: see text] = 0.0531), CPR (61.54% vs. 44.44%; [Formula: see text] = 0.0531), and LBR (61.54% vs. 33.33%; [Formula: see text] = 0.0014) were reported. Multivariate analysis on ANOVA suggested, comparable pregnancy rates at Day 5 and Day 6 ([Formula: see text] = 0.728). Conclusions: Day-5 euploid blastocysts seem to offer better reproductive outcomes than Day-6 euploid blastocysts. Further research is recommended to evaluate the reproductive outcomes of Day-6 blastocysts.