Recent scientific advances in the understanding of the dynamic physiology of early human embryos have led to the development of in vitro culture systems capable of yielding viable blastocysts with greater consistency. A blastocyst is a many-celled embryo in the stage just prior to hatching, approximately five days after fertilization. These blastocyst embryos have a much greater likelihood of implanting in the uterus. Improvements in our culture medium have given us the ability to allow embryos to metabolize more efficiently, and therefore we can maintain them outside of the human body for longer periods of time.
Growing embryos in vitro to the blastocyst stage (day 5-6) for assisted reproduction offers several advantages over the transfer of cleavage stage (day 2-3) embryos. These advantages include:
- A higher implantation rate
- A decrease in the number of embryos transferred
- The opportunity to select more viable embryos for transfer
- Better synchronization between embryo and endometrium at the time of embryo transfer
- A longer time in culture that provides the opportunity to perform pre-implantation genetic diagnosis (PGD) when such is indicated
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 collection of cells called the “inner cell mass” will go on to form the fetus. Blastocysts require at least five days to develop, and are generally heartier than three-day-old embryos. Only the strongest embryos survive to the blastocyst stage, so we implant the strongest embryos possible.
However, blastocyst transfer is not appropriate for all IVF cycles. There are some patients that will be better served by traditional day 3 embryo transfer. 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 the embryos for the additional days. For example, if a couple has only two or three embryos on day 3, one risks losing all viable embryos by waiting until the blastocyst stage. On the other hand, if a couple has six embryos on day 3, we would be more likely to wait for blastocyst development. Our physicians at The Center for Advanced Reproductive Medicine & Fertility will discuss blastocyst transfer with all appropriate IVF patients.