What is a blastocyst?

After egg retrieval, the fertilised egg may develop into an embryo over a period of three days in the laboratory. The embryo typically divides into between 6-8 cells. A blastocyst, by comparison, is an embryo that has advanced to the five or 6-day stage; which means the embryo has divided many more times into many more cells over this period. Blastocysts have a very thin outer shell thus potentially increasing the odds of implantation into the uterine cavity. Most of the blastocyst contains a fluid cavity and it is possible to see the cells which will become the baby and those which will make up the placenta. While the majority of fertilised eggs will develop into a three-day old embryo, only perhaps 40% of these embryos will develop into a blastocyst. Therefore, blastocysts are considered to be a more “select” group of embryos with a higher chance of pregnancy.


Day 3 embryo	Blastocyst