Depending on the individual situation of the couple, the embryos can be transferred either on day 2-3 or, after extended embryo culture ("blastocyst culture"), on day 5-6 after having been retrieved.
On day 2, the embryos normally consist of 2-4 cells, and on day 3, they have grown to 4-8 cells. But these "young" embryos are not all the same. By evaluating the speed and continuity of embryo development, embryologists can draw some conclusions about the likely ability of the embryos to develop further.
Improved cultivation has made it possible to keep the embryos in culture for 5-6 days without undue stress. Ideally, their cells continue to divide, reaching a stage where they look like a small berry (morula) and develop an interior cavity (blastocyst). By examining the blastocyst, it is possible to tell which part is going to form the actual embryo and which the amniotic cavity. Embryos in the morula or blastocyst stage are nearly ready to implant in the uterus. However, not all embryos make it to that stage - depending on the parents' age and other factors, the average ratio is 30-50%. The others stop developing at an earlier stage and have no chance to implant.
Blastocyst culture is legal in Germany. However, according to the Embryo Protection Act, we are obliged to avoid "the planned storage of embryos", i.e. embryos should only be cryopreserved in emergencies. Therefore, we must bring all our experience to bear to decide in each case how many pronuclear oocytes should be cultivated to the blastocyst stage in order to be able to transfer the desired one or two viable embryos. We are obliged to document the reasons for this decision in each individual case.