Five to six days after the egg recovery embryos reach a stage called "Blastocyst". With the advancement in the culture medium it is possible to grow embryos to this stage. By leaving the embryos in culture medium for longer, the more viable embryos will become evident. So pregnancy rate can be improved over the transfer of similar numbers of embryos on day 2 or 3. Blastocyst transfer is also useful where only one embryo is electively transferred.