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Sometimes more embryos are produced in an IVF cycle than can safely be transferred to the uterus because of concerns about multiple births. When this is the case, it may be possible to freeze the extra embryos for use in a future IVF cycle in a process known as cryopreservation. Only embryos that are considered to be viable are frozen or cryopreserved for future use.
 Cryopreservation involves gradually freezing embryos in a variety of special media. Embryos can be frozen because of their relatively strong cell membrane, the zona pellucida (glycoprotein coating), that surrounds an embryo and helps to protect the embryo during the cryopreservation process. Although eggs have a zona pellucida similar to that of an embryo, eggs have a much higher water content that makes them more fragile when frozen because the water tends to form ice crystals causing cell (egg) rupture. Human eggs have been successfully cryopreserved but this is still considered an experimental procedure reserved for women who will undergo therapies (cancer chemotherapy or radiation) that will likely destroy their ovaries and eggs.
Embryos must be carefully thawed in a variety of sequential media to prevent cell rupture. The maximum length of time an embryo can remain viable when frozen is not known, but we encourage couples to undergo a frozen embryo transfer (FET) within 5 years from the time that the embryo(s) was frozen. Successful pregnancies have resulted from embryos over twelve years old.
Approximately 1 in 4 couples undergoing IVF will have an excess number of embryos that may be cryopreserved (on culture Day 5-6). While embryos can be frozen at various stages during an IVF cycle (day 1, day 3, etc.), our usual strategy is to freeze at the blastocyst stage (Day 5-6). Pregnancy rates using frozen and thawed embryos are usually lower than those with fresh embryos.
The major advantages to using cryopreserved embryos is simplified treatment and reduced infertility treatment costs. The patient only needs to undergo uterine preparation by way of hormone replacement and an embryo transfer to complete a FET. Since the embryos are already available, it is not necessary to undergo a stimulation cycle with fertility drugs and monitoring thus lowering infertility treatment cost. Oftentimes, there are additional embryos for freezing from donor egg cycles.
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