Five days after fertilization, the embryo becomes a blastocyst. It has approximately 128 cells and a cavity full of fluid. Many embryo transfers in in-vitro fertilization (IVF) today are performed in the blastocyst stage. The human embryo typically will divide into six to eight cells three days after fertilization. Further division will normally result in a blastocyst five days after fertilization. The blastocyst is graded based on its appearance under the microscope. The best blastocysts are typically transferred to the patient’s uterus. Others may be cryopreserved (frozen) for future implantation if they are viable.

The advantage of transferring a blastocyst (day-five embryo) is that further culture in the laboratory selects the most viable and hardy embryos. Such embryos theoretically are more likely to survive and result in a pregnancy. Therefore, the number of embryos transferred may be reduced, resulting in a similar or better pregnancy rate than with day-three embryos, but with lower multiple pregnancy rate. At Bennett Fertility Institute (BFI), most embryo transfers are performed with blastocysts on day five.