A blastocyst is a ball of cells that forms early in pregnancy, about five to six days after a sperm fertilizes an egg. It is a rapidly dividing ball of cells, with the inner group of cells becoming the embryo. The blastocyst is a crucial stage of embryonic development and plays a significant role in a successful pregnancy. It is composed of two already differentiated cell types, the inner cell mass and the enveloping layer. The inner layer is about three to four cells thick, and the outer layer is about one cell thick. The cells in a blastocyst divide rapidly, and a mature blastocyst may contain as many as 200 to 300 cells. The blastocyst stays in the uterus for several days before it implants in the inner lining of the uterine wall. About 10 to 12 days after fertilization, the blastocyst develops into an embryo, which remains an embryo until about nine weeks after implantation, when it then becomes a fetus. The primary benefit of using blastocyst stage embryo culture is that fewer embryos need to be transferred into the patient because each day 5 embryo has a higher chance of implanting than a day 3 embryo does.