Within 5-6 days of the oocyte retrieval, the embryos reach the developmental stage of blastocysts. Blastocyst is formed of two distinct cell types. The surface cells that surround the blastocoel cavity will later develop into the placenta. A more centrally located group of cells - the inner cell mass, will become the fetus.
Today, it is considered that embryos should be transferred within the third and fifth day of the oocyte retrieval. If the patient has at least four first-degree quality embryos, a fifth day transfer could also be made. Considering all factors including the genetics and the quality of the environment in which the culture grows, the chances of an embryo reaching the Blastocyst stage is 40-50 percent.