An embryo that has developed for five to seven days after fertilization and has 2 distinct cell types and a central cavity filled with fluid (blastocoel cavity). The cells in a blastocyst have just started to differentiate. The surface cells that surround the cavity (just under the outer shell) are called the trophectoderm and will later develop into the placenta. A more centrally located group of cells the inner cell mass, will become the fetus. The blastocyst usually forms on day 5 as fluid builds within the compacted morula .A healthy blastocyst often begin hatching from its outer shell, called the zona pellucida between day 5 to 7 after fertilization. Within 24 hours after hatching, embryo implantation after IVF (or a “natural” pregnancy) begins as the embryo invades into the uterine lining. The blastocyst releases HCG hormone (the pregnancy test hormone) which leaks into the mother’s blood as the embryo implants.

Blastocyst transfer is not an option for all IVF patients. The technique is most successful with patient who are younger and have a large number of eggs available at retrieval. On average, patient with six or more high quality embryos on day three are the best candidates for blastocyst culture there is a better chance of more embryos growing successfully to day five.