The embryo transfer usually takes place on the second or third day after egg retrieval, when the embryos, on average, are between four and eight cells.
At times, depending on each case, it may be recommendable to keep the embryos in culture for a few more days (until six days after the retrieval) with a view to observing their development and selecting the embryos that are truly viable in order to increase the possibilities of pregnancy. On average, two out of every three embryos continue to develop normally until the sixth day, forming an embryonic structure known as blastocyst. This embryo is considered highly evolutionary and usually has an average of between 50 and 100 cells. Therefore, at this level, the transfer of more than two embryos is never recommended since the multiple pregnancy rate is quite high.
On the other hand, the embryos that do not reach this advanced stage (the remaining two thirds) usually block and stop dividing. The pregnancy rate increases slightly in accordance with the transfer of embryos that have been in culture for a lower number of days.