Grades of expansion of a blastocyst

Depending on its expansion, the blastocyst can be classified as early, cavitated, expanded, or hatching. The early blastocyst is the one that is initiating cavitation, i.e., the blastocele begins to form to give rise to the complete blastocyst structure. When it is cavitated, the blastocele has already formed, but it must continue to fill with liquid so that the blastocyst can expand. When the blastocyst is expanded, its size is larger and the zona pellucida is thinner, so hatching, i.e. the exit from the zona pellucida, can begin. Once it has hatched, implantation can take place.