Fertilization of the egg occurs in the fallopian tube. The initial cell divisions of the fertilized egg also take place there and result in an embryo of 4 to 8 cells. As the newly-formed embryo continues to develop, it moves toward the uterine cavity where additional cell divisions result in formation of the “blastocyst”, an embryonic stage characterized by up to 150 cells and having the appearance of a hollow ball. When the embryo reaches the "blastocyst" stage, it is ready to implant.
“Blastocyst culturing” became an important part of IVF treatment when, in the late 1990s, sequential embryo culture media were developed to approximate the changes the embryo encounters in going from fallopian tube to the uterus.
The advantage of blastocyst culture in IVF is that pregnancy may be established with the transfer of 1 or 2 blastocysts compared to the 2 or more embryos that are more typically transferred on the 3rd day after fertilization at the 4- to 8-cell stage of development. The transfer of fewer embryos can reduce the chance for multiple pregnancy and the associated complications that sometimes arise.