Blastocyst stage embryo culture is used as a tool to select the highest quality embryo(s) for replacement. This technique is of particular value in cases where there are numerous good quality embryos developing on day-3 of culture. Human embryos reach the blastocyst stage of development at 5 days following egg retrieval. Those embryos that have survived and grown to this stage of development have a high implantation potential. By extending culture from day-3 to day-5 of development, an embryologist can better select the most viable embryo from transfer.

The primary benefit to the patient of using blastocyst stage embryo culture is that fewer embryos need be replaced into the patient while still maintaining a high pregnancy rate. This results in a greatly reduced risk for twin or triplet pregnancies.