Is day 5 blastocyst better than day 6 blastocyst? Evidence from NGS-based PGT-A results

Background Aneuploidy is the principal genetic factor leading to the failure of embryo implantation. For most patients who accept the non-preimplantation genetic testing (PGT) cycle, non-invasive methods to select euploid embryos with the best pregnancy potential are desirable Methods This retrospective study recruited women undergoing PGT for aneuploidy (PGT-A) with trophectoderm biopsy from January 2019 to December 2020. The ploidy status of embryos was determined by next generation sequencing (NGS). Results Altogether 2531 blastocysts from 839 PGT-A cycles were evaluated. The euploid rate of day 5 blastocysts seemed to be significantly higher than that of day 6 blastocysts, either from the same ovarian stimulation (OS) cycles (49.9% vs 35.7%, P < 0.001) or from different OS cycles (48.2% vs 27.8%, P < 0.001). Both the younger maternal age (adjusted OR = 0.917, 95% CI: 0.892–0.944, P < 0.001) and day 5 stage (adjusted OR = 1.735, 95% CI: 1.415–2.127, P < 0.001) were independently associated with the greater euploid rate of blastocysts. However, after single euploid embryo transfer, the clinical outcomes of day 5 blastocysts were comparable to those of day 6 blastocysts, no matter whether they were from the same OS cycles or not. Conclusions Our results revealed that day 5 blastocysts possess a higher euploid rate than day 6 blastocysts independent of the OS cycles. Giving priority to a day 5 blastocyst over day 6 blastocyst will increase the likelihood to select single euploid embryo for transfer in non-PGT cycles.