Over the past decade, improvements in culture conditions have allowed the possibility of extended embryo culture until day 5, which has been increasingly preferred to a transfer at day 2 ( cleavage stage) or day 3. In fact, day 5 blastocyst transfer is considered more physiological compared with transfers at day 2 or day 3.
It theoretically enables a better synchronization between the embryonic stage and the endometrial receptivity.
Moreover, it is well known that not all embryos obtained at day 2 develops to day.
Microscopic appearance at day 2 mostly isn't enough to predict their implantation potential .
To select embryos at day 2 or day 3 are insufficient to predict their developmental potential, notably because, in many cases, they fail to discard chromosomally abnormal embryos