The decision of the best moment to transfer the embryos will depend on the situation of each patient. It is true that embryos at day 2 of development are not usually transferred because they are embryos with little information. In addition, embryos at day 4 of development, the embryos are in the morula stage and there is no standardized evaluation.
Therefore, it is most common to transfer embryos on day 3 or day 5 of development. The choice of one day or the other will depend on medical criteria, the characteristics of the embryos, the number of embryos, whether PGD is performed, etc.