Expanded Blastocyst

An expanded blastocyst is a zygote that has developed for around 5 days and has started the process of differentiation. There are three main components to a blastocyst. The inner cell mass layer of cells develops into the fetus, the trophectoderm or trophoblast develops into the placenta and the blastocoel is an empty cavity of fluid.

As the blastocyst develops, the blastocoel cavity enlarges with fluid which causes the protective shell known as the zona pellucida to thin. The degree to which this expansion has occurred is said to be how expanded the blastocyst is.

The degree of expansion of the blastocyst is a consideration in grading embryos. With in vitro fertilization (IVF), ideally only the best embryos, which are the ones assigned the best grades by an embryologist, are selected for transfer.