Blastocyst

A blastocyst is an embryo that has developed for roughly five to six days after fertilisation and formed a fluid-filled cavity, an inner cell mass that may become the fetus, and an outer layer, the trophectoderm, that may become the placenta. Reaching this stage helps the laboratory select between embryos, and it is the stage at which most transfers, freezing and biopsies are done.

Also known as: blast, day 5 embryo, blastocysts

Last reviewed: 8 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.