The surface cells that surround the cavity (just under the outer shell) later develop into the placenta and the more centrally located group of cells are called the inner cell mass, that develop into the foetus.
In a conventional IVF, the embryos are transferred to the uterus after 48 hours between the fourth and eight cell stages. In a natural conception, the embryo reaches the uterine cavity only on the fifth day. With the availability of an extended culture medium, we can culture the embryos in the laboratory till the blastocyst stage (i.e. fifth or sixth day). This procedure is called as blastocyst culture. With the blastocyst embryo transfer, we can transfer the embryos of a smaller number, while keeping the overall pregnancy rates high. This reduces the risk of multiple pregnancies.