Embryonic development in the first few days of the embryo's life is very fast. The embryos that follow a normal development, on the third day of life have about 8 cells and, on the fifth day of life, they are already a structure called a blastocyst, which has more than 200 cells, with an internal cell mass (the part that will form the baby) and a trophectoderm (the part that will form the placenta) already defined.
During this development process, a first selection already occurs in the laboratory itself, since not all embryos reach the blastocyst stage. In fact, some embryos that show good characteristics on day 3 do not reach day 5 (they are blocked or their quality is not good enough).
In this sense, we have more information on embryo quality on the 5th day of development and we can better select which embryos are most likely to give an evolving pregnancy. For this reason, the current trend in most assisted reproduction centres is to transfer on the fifth day of development.