The collected oocytes, inseminated or microinjected, are incubated at 37°C and under appropriate gazeous conditions in a specific medium.
Twenty hours later, microscopic examination is performed to determine which eggs have been fertilized (presence of two "pronuclei") (Fig. 8-3). Fertilized eggs are then transferred into a new culture medium
and put back into the incubator. This procedure is aimed at eliminating supernumerary, dying spermatozoa which could otherwise unfavorably modify the culture medium.
Thirty or forty hours later, after sperm penetration into the oocyte, the first cell divisions leading to the early embryo will start.

The embryos are replaced into the uterus when they have reached the 4 to 8 cell stage (early embryonic cells = blastomeres) or even at a more advanced stage (blastocyst) if necessary. (Fig. 8)

CHU Saint-Pierre, Université Libre de Bruxelles
UMC Sint-Pieter, Vrije Universiteit Brussel
Saint-Pierre University Hospital, Free University of Brussels