At the blastocyst stage of development, the embryo is a hollow ball. – the thin, outer layer is called the “trophectoderm”, which later will form the placenta and other supportive membranes. A blob of cells called the “inner cell mass” are the cells that will go on in development to form the fetus itself. Blastocysts usually require at least five days to develop, and are “heartier” than day 3 embryos. Natural selection works to insure that only the “strongest” embryos survive to the blastocyst stage.
Because blastocysts have a higher implantation rate than day 3 embryos, fewer are transferred to the uterus, reducing the incidence of multiple births. They also have a higher implantation rate than day 3 embryos.
However, blastocyst transfer is not appropriate for all IVF cycles/couples. As the embryos develop beyond day 3, invariably some do not survive the extended culture. There must be enough viable embryos on day 3 to risk culturing for the additional days. For example, if a couple has only 2 embryos on day 3, we would not risk loosing all viable embryos by extending the culture to the blastocyst stage. On the other hand, if a couple has 6 embryos on day 3, we would be more likely to wait for blastocyst development.
Our physicians discuss blastocyst transfer with all appropriate IVF patients.