The blastocyst is the stage of the embryo after 5 or 6 days of development in the cell division process and consists of more than 100 cells. Under natural conditions, it is precisely at this stage that the embryo is ready to implant in the endometrium.
However much we strive to recreate conditions similar to the human body, not all embryos manage to reach that stage in the laboratory. Depending on the quality of oocytes, spermatozoa, and other factors, only a few of them will succeed in developing to the blastocyst stage. Using newly developed culture media, a higher implantation rate has been observed following transfer of blastocysts (60%), compared to day 3 (40%) embryo transfer.