The blastocyst is an embryo that has developed during 5-7 days after fertilization and has developed 2 cell types and a central cavity full of liquid called the blastocele. The cells of the blastocyst have begun to differentiate. The cells of the surface around the cavity are called trophoblast and later will become the placenta. The cells in the central zone are called the internal mass and the fetus develops from these cells.

The blastocyst culture and embryo transfer on day 5 helps select the highest quality embryos to transfer. It is only performed in special cases.