Embryo cryopreservation is the process of freezing and storing embryos and is a part of most in vitro fertilization programs. The process is chosen for reasons as varied as providing an additional chance for pregnancy or saving embryos in the phase of certain medical treatments.

The embryos first go through cryopreservation, which removes water from the cell and replaces it with cryoprotectant. Then the embryo is usually frozen through vitrification; this preserves the embryo without forming damaging ice crystals. The embryos are stored in liquid nitrogen.

In vitro fertilization and egg donation cycles often result in more embryos than actually needed for transfer.  After the transfer of fresh embryos into the uterus, any remaining embryos which are suitable for freezing are cryopreserved.  Embryos may be frozen at the day-1, day-3 stage or the day-5 or day-6 blastocyst stage.  A frozen embryo transfer may then be performed at any future time in a simple short procedure.

There are various indications for embryo freezing beyond the storage of surplus embryos.  In poor responders (patients who respond to injectable hormones with only a small number of eggs/embryos) for example, it may be necessary to perform several cycles of IVF/embryo freezing in order to accumulate enough embryos for a single embryo transfer.  Similarly, in patients who require a combination ZIFT/IVF(see above), embryo banking may be necessary in order to obtain enough embryos for transfer both into the fallopian tubes as well as the uterus.  Occasionally, a uterine abnormality may preclude the transfer of fresh embryos, requiring storage of embryos until the uterus is normalized.