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.