The blastocyst is a structure formed in the early development of mammals. It possesses an inner cell mass (ICM) which subsequently forms the embryo. The outer layer of the blastocyst consists of cells collectively called the trophoblast. This layer surrounds the inner cell mass and a fluid-filled cavity known as the blastocoele. The trophoblast gives rise to the placenta. The name "blastocyst" arises from the Greek βλαστός blastos ("a sprout") and κύστις kystis ("bladder, capsule"). In humans, blastocyst formation begins about 5 days after fertilization, when a fluid-filled cavity opens up in the morula, a ball consisting of a few dozen cells.The blastocyst has a diameter of about 0.1-0.2 mm and comprises 200-300 cells following rapid cleavage (cell division). After about 1 day (5–6 days post-fertilization), which is the time usually required to reach the uterus, the blastocyst begins to embed itself into the endometrium of the uterine wall where it will undergo later developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, which prevents it from adhering to the oviduct as it makes its way to the uterus.The blastocyst is completely embedded in the endometrium only 11–12 days after fertilization. The use of blastocysts in in-vitro fertilization (IVF) involves culturing a fertilized egg for five days before implanting it into the uterus. It can be a more viable method of fertility treatment than traditional IVF. The inner cell mass of blastocysts is also a source of embryonic stem cells.

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