What is a Blastocyst?
This is an embryo, which has been growing for at least five days. It is more developed than a day three embryo, and consists of a larger number of cells (approximately 60) that are to form a tiny fluid filled ball. There are now two cell types present, an inner cell mass destined to become the fetus, and an outer layer of cells that will attach to the uterine lining and form the fetal part of the placenta. This is the stage when the embryo would normally arrive in the uterus during a natural conception.