Why is it necessary to wait until day 5 of embryo development to perform the biopsy with PGS/PGT-A/CCS?

On day 5 of development, embryos reach a stage termed blastocyst. During this stage, a process called cell differentiation occurs, besides other processes. Up to this moment, all embryo cells are the same and have the same capacity. Yet, through cell differentiation, they specialise in forming, on the one hand, the amniotic sac and placenta (these cells form the trophectoderm) and, on the other, the embryo (these cells form the inner cell mass). At this stage, the cells that will not be producing the foetus can already be differentiated and selected. Hence, their biopsy will not damage the embryo. In the past, embryo biopsies used to be performed at earlier stages of development, such as day 3, but it has later been shown to affect negatively the capacity of embryos to implant. In contrast, recent studies have proved that it is much safer to perform biopsies on day 5.