What is the reproductive potential of euploid day-6 and day-7 blastocysts compared to euploid day-5 blastocysts?

To determine whether the reproductive potential of euploid embryos differs by day of blastulation in frozen embryo transfer cycles. We conducted a retrospective cohort analysis of frozen blastocyst euploid embryo transfers (FET) between 1/2018-1/2023 at a large, multicenter private fertility practice. Autologous, single, euploid FETs were included. Patients with a diagnosis of recurrent pregnancy loss and uterine factor were excluded. To determine differences between groups, parametric and nonparametric analyses were performed using independent sample t-test and Mann-Whitney U test. Generalized estimating equation (GEE) modeling was used to control for age, BMI, prior number of transfers, prior number of losses, and embryo grade. Transfers were categorized according to the embryo day of blastocyst development. The analysis included 13,830 euploid embryo transfers (8363 Day 5 (D5) euploid embryo transfers, 5077 Day 6 (D6) euploid embryo transfers, and 390 Day 7 (D7) euploid embryo transfers).Clinical pregnancy rate per FET was 66% for D5 euploid embryos, 54% for D6 euploid embryos, and 32% for D7 euploid embryos (p<0.001). The live birth rate per FET was 55% for D5 euploid embryos, 44% for D6 euploid embryos, and 21% for D7 euploid embryos (p<0.001). Miscarriage rates were not significantly different between D5, D6, and D7 euploid embryo transfers. Routine culture through day 7 results in usable euploid blasts and subsequent live birth of day 7 embryos. While the clinical pregnancy rate and live birth rate of day 7 euploid embryos is significantly lower, embryos taking longer to reach the blastocyst stage, once determined to be euploid, still achieve reproductive potential.