After egg collection, sperm and egg fertilize for 3 to 5 days in a special culture medium. During this time, embryologists monitor the growth of each embryo to assess its viability. The strongest and most viable embryos will continue to grow through Day 5 to form a Blastocyst. The weaker embryos, though may have appeared to have a strong growth pattern on Day 3, may slow and stop growing by Day 5. If there are fewer embryos on Day 3, we may choose to transfer the embryos to the wife's uterus on Day 3 as the uterus is the best incubator for the embryos.What are the advantages of Blastocyst, or Day 5, Transfer?
Blastocyst Transfer is most advantageous for couples with a large number of embryos. Allowing the embryos to grow through Day 5 allows for the highest quality of embryo to be selected for transfer while reducing the rate of multiple pregnancies as fewer embryos of higher quality will be transferred.