Authors: Tricia Leigh Adams, MS, Mariah L Markle, D.V.M. and Chelsey Audra Leisinger, PhD, Ovation Fertility, Baton Rouge, LA
Objective: Day 5 and Day 6 biopsied embryos were evaluated to compare implantation rates (IR) and ploidy rates (P-Rt) to provide support for embryo selection. Previous studies have been
inconclusive when comparing superiority of Day 5 or Day 6 embryos for transfer. Some reports have indicated that Day 5 embryos yield higher pregnancy rates, while others have noted trends with no
statistical significance showing increased success of Day 5 compared to Day 6. Notably, these effects were no longer present when euploid embryos were used for transfer. Additionally, Day 6 embryos have been associated with increased aneuploidy rates.
Materials and Methods: Retrospective analysis of IR of 224 transferred euploid embryos and P-Rt of 1431 embryos from two IVF labs in the United States between 2019-2021. Embryos were
subjected to preimplantation genetic testing (PGT) on Day 5 or Day 6 followed by vitrification. Implantation rate was defined as the presence of an intrauterine implantation sac with the detection
of a fetal heartbeat on ultrasound. The following categories were included in analysis of P-Rt: Normal (euploid), Abnormal (aneuploid), Low-Mosaic (LM), and High-Mosaic (HM). Trophectoderm
biopsies of Day 5 and Day 6 embryos were evaluated by NextGen Sequencing at the same genetics laboratory (Ovation Genetics, Nashville, TN). Euploid embryos were transferred in subsequent
frozen embryo transfer (FET) cycles and monitored by clinical ultrasound. Chi-square statistical analysis was performed to identify potential differences (p<0.05).
Results: Transfer of euploid Day 5 embryos significantly increased IR compared to euploid Day 6 embryos. The IR of Day 5 embryos was 62% compared to 40% for Day 6 embryos (p=0.0026). No
significant difference was observed in P-Rt between Day 5 or Day 6 biopsied embryos (p=0.082). The rate of euploid embryos was 61% versus 54% for Day 5 and Day 6, respectively. The aneuploid
rate of Day 5 embryos was 21% compared to 27% in Day 6 embryos. The LM rate was 9% in Day 5 versus 11% in Day 6, with a Day 5 HM rate of 9% compared to a Day 6 rate of 8%.
Conclusions: Transferring euploid Day 5 embryos resulted in higher IR versus Day 6, and no difference in P-Rt was observed between the two groups.
Impact Statement: The present study provides useful information for embryo selection according to the day of biopsy and vitrification in clinics that utilize a high percentage of PGT and subsequent FET. Previous studies utilized varied parameters that were difficult for comparison. Recommendations to transfer Day 5 over Day 6 may have resulted in disposition of viable embryos
with the ability to result in live birth. The inclusion of a larger data set could strengthen the recommendation of the superior day of transfer. Embryo selection is a multifactorial process encompassing a range of parameters and further investigation is warranted.