Similar implantation competence in euploid blastocysts developed on day 5 or day 6 in young women: a retrospective cohort study.

The results from different studies are inconsistent regarding whether development potential correlated with embryo development speed after single euploid blastocyst transfer. The age-associated reproductive decline is not only because of the difference in aneuploidy rates but also because of metabolic and epigenetic changes of the embryos. Therefore, we aimed to assess the independent effect of embryo development speed on implantation potential in young women. A total of 326 young women who underwent preimplantation genetic testing for monogenic diseases with aneuploidy screening were analyzed. Day-5 and day-6 euploid blastocysts yielded similar implantation rates (65.20 vs. 61.22%). The odds ratio (OR) remained non-significant after adjusting for confounders (adjusted OR = 0.84, 95% confidence interval 0.52-1.36). There was a trend that day-6 euploid blastocysts had a higher miscarriage rate (13.33 vs. 9.20%). However, the live birth delivery rate of day-5 blastocysts was similar to that of day-6 blastocysts (59.20 vs. 53.06%). In the stratified analysis, live birth delivery rates were similar between day-5 and day-6 similarly graded euploid blastocysts (excellent and good, 62.04 vs. 64.71%; average, 58.73 vs. 53.70%; poor, 43.75 vs. 44.44%). Embryo development speed has no obvious impact on implantation competence in young women's vitrified/warmed euploid embryo transfer cycles.