A blastocyst is an embryo that has developed to the stage where it has two different cell types and a central fluid-filled cavity. The surface cells, called the trophectoderm, will become the placenta, and the inner cells, called the inner cell mass, will become the fetus itself. Blastocyst formation in the human usually occurs on the 5th day after fertilization. Unlike embryos that have developed for 3 days (which usually have 6-8 cells), blastocysts typically have > 200 cells.

 

Example of a blastocyst

Example of a hatching blastocyst

Not all embryos are capable of progressing to the blastocyst stage of development. Some embryos are not capable of existing in our incubators beyond 3 days post-fertilization. For this reason, embryos are usually transferred into the uterus after three days of culture.

Blastocyst transfer may improve the chances of implantation in some patients. Embryos that have developed for three days (day 3 embryos) would normally be growing in the fallopian tubes, not in the uterus (embryos remain in the fallopian tubes for about 4-5 days when conception occurs “naturally” or “in vivo”). Thus, in vivo, the implantation process occurs after blastocyst formation and hatching have occurred. Thus, by transferring blastocysts, we may be able to better simulate what normally occurs at the time of embryo implantation in a woman’s uterus.

Transferring blastocysts following IVF may also reduce the chances of multiple pregnancies. By culturing embryos to the blastocyst stage, fewer embryos are needed to be transferred in order to achieve a high pregnancy rate. Thus, fewer blastocysts can be transferred (one to two) compared to the number needed to be transferred on day three (often three or more) in order to maximize the chances of pregnancy while reducing the risk for multiple pregnancy.