The German Embryo Protection Act restricts the number of embryos that may be implanted into the uterine cavity. This can be 1, 2 or 3 embryos. The decision for this must be made one day after the egg collection, in the so-called pronuclei stage (pronuclei=PN).
The purpose of this regulation is that the legislator wants to avoid an accumulation of embryos that are not used.
An advantage from the prolonged culture to the blastocyst stage is only given if many oocytes have been fertilized.
If this is the case, it may also be advantageous to further culture several cells in the pronuclear stage (for example 4 PN) - up to the blastocyst stage. The number of cells to be further cultured is determined individually for each patient. The advantage is that naturally not all cells survive these first 5 days.
For one patient, 10 oocytes can be punctured. In the course of IVF ICSI therapy, 5 cells in the PN stage are produced one day later (day P+1).
All cells are further cultured and again one day later (day P+2) 4 cells have emerged at the two and four cell stages. These embryos are now transferred to the extended culture and one day later (day P+3) develop into 3 cells in the eight-cell stage. Again, one day later (day P+4) 3 cells are found in the 16-cell to berry stage (morula stage). Often the final selection now takes place here, so that on day P+5 2 cells are in the berry stage to vesicle stage (blastocyst) and are then also transferred.
In the blastocyst stage we distinguish the early from the normal and expanded blastocyst. In the latter stage, the embryo is about to "hatch" ("hatching") from the oocyte shell. After hatching, the embryo may implant in the mucosa of the uterus. Before each embryo transfer, the embryos are subjected to a precise quality control.