A blastocyst that develops into a blastocyst on the seventh day, or D7 for short, is a blastocyst that has a seventh day. Blastocysts are mainly associated with implantation, and human embryos are mainly implanted into the mother in the form of blastocysts, which usually form on the fifth to seventh day after fertilization of the egg. The number of days of blastocyst formation is an important indicator of its developmental potential. Day seven blastocysts are not of particularly good quality due to their slower development and poorer developmental potential than day five or six blastocysts, as well as their larger size, and therefore have a relatively lower success rate for transfer. Cultivation and transfer of blastocysts need to be operated in regular hospitals to ensure the success rate and safety to a large extent.