Blastocyst

A blastocyst is a stage of embryo development, usually reached on Day 5 or occasionally Day 6, in which the embryo has formed a fluid-filled cavity and organised into two distinct groups of cells: the inner cell mass, which contributes to the future baby, and the trophectoderm, which contributes to the future placenta. Reaching the blastocyst stage in the laboratory is one useful sign that an embryo has continued developing, but it is not the only factor that matters for pregnancy. An embryo described as "expanded" has grown larger and thinner-walled as the blastocyst matures further. Not all embryos that start dividing reach the blastocyst stage, and this varies between cycles; your embryology team can explain what was observed in your specific cycle.

Also known as: Day-5 blastocyst, Day-6 blastocyst, Day 5 blastocyst, Day 6 blastocyst, expanded blastocyst

Last reviewed: 9 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.