Standard IVF-embryo transfer involves transfer of embryos at the cleavage stage, after the embryos have been cultured (grown) in the laboratory for 2 or 3 days (2 or 3 days after egg retrieval). Following fertilization, each day 1 embryo (zygote) is formed of a single cell. As the embryo grows in the laboratory, cellular division occurs, and the cell splits into two daughter cells (blastomeres). With further division, each daughter cell splits into two more daughter cells, and so on... Therefore, after 3 days of being cultured, each embryo normally contains between 6 and 8 cells. This is the stage at which most embryo transfers are performed.
An alternative to transferring embryos on day 3 is the blastocyst transfer. Blastocysts are embryos which are more advanced in their development, typically 5 or 6 days old (5 or 6 days after egg retrieval). Blastocysts have a supernumerary number of cells (between 64 and 128 cells per blastocyst). Because they are more advanced, blastocysts also have a better implantation rate than day 3 embryos. As such, a smaller number of blastocysts can be transferred to achieve similar or greater pregnancy rates than with the transfer of a higher number of day 3 embryos. When concerns arise regarding higher order multiple gestation (triplets or more), blastocyst transfer should be considered.