In an IVF cycle, embryos are cultured until mature and ready for transfer, which is usually from three to six days. As the embryos develop, they pass through several developmental stages including blastocyst formation. (We typically refer to day 5 embryos as blastocysts, however, embryos may form blastocysts before or after 5 days.)
A blastocyst is a developing embryo that has differentiated (or separated) into two distinct cells types. The surface cells are termed the trophectoderm and will eventually become the placenta and the inner cell mass will become the fetus. A healthy blastocyst should hatch from its shell (zona pellucida) by the end of six days or earlier and begin to implant within the lining of the uterus.
Blastocysts are typically “stronger” than day 3 embryos. In Darwinian terms, only the strongest, and most fit, embryos will survive the additional two days in culture. Blastocysts have a higher IVF implantation rate and are more likely to survive than day 3 embryos. Because of this increased viability, fewer blastocysts need to be transferred in the IVF cycle thus dramatically lowering the rate of high order (>2) multiple births.
However, even with the advantages of increased viability and lower multiple birth rates, blastocyst transfer is not for every patient(cycle). The longer the embryos are cultured the fewer the embryos that will remain viable for transfer. For example, there are usually more embryos on day 1 than on day 3 or day 5 as some embryos stop growing or their growth slows during the culturing process. Our fertility doctors discuss the possibility of blastocyst transfer with each IVF patient.
There must be enough viable embryos on day 3 to “risk” culturing to day 5. For example, if only two embryos are present on day 3, one or both, could stop growing by extending culture to 5 or 6 days, which would result in the loss of the cycle. On the other hand, if 6 good embryos are available on day 3, the chances are very good that 2 or more will survive to day 5 making blastocyst transfer feasible. Whether or not the patient plans to cryopreserve some of their embryos will also influence the decision on whether to extend the culture.
Sometimes, more blastocysts develop than can be safely transferred. Some women donate their extra embryos to other patients through our "Embryo Donation Program".