As part of the process of in-Vitro fertilization ("IVF"), drugs are administered to the woman to stimulate her ovaries to produce many eggs. These eggs are then recovered and as many as possible are fertilized with sperm to allow for the subsequent development of embryos. When the developing embryos are ready to be transferred, some or all of them are placed into the woman's uterus so that they may implant and develop into a viable pregnancy.

Depending on the number of embryos that were transferred to the uterus, this may leave a number of additional embryos. These embryos may be preserved by cryopreservation (freezing) so that they may be transferred into the uterus in a future treatment cycle. Cryopreservation involves a series of steps to gently remove water from the cells, freeze them, and store them in liquid nitrogen at -196 degrees Centigrade. If it is decided that they will not be cryopreserved, the embryos will be disposed of, unless the patient consents to donate them for research. Embryo freezing allows women to conceive in a future cycle without undergoing further ovarian stimulation and egg retrieval. There is no known time limit as to how long an embryo can be kept frozen, and still result in a successful pregnancy.

Whenever it is decided to implant the embryos, they are slowly thawed, water is restored to the cells, and they are incubated with nutrients to allow further development prior to transfer. It is possible that some of the embryos will not survive the freeze, store and thaw cycle. It is only possible to determine which embryos have survived once they have been thawed.

Based on studies that have been performed in both animals and humans, the risk of birth defects in babies that develop from frozen embryos is no higher than in maternal age-matched naturally conceived pregnancies. The age-related risk of birth defects in babies that develop from frozen embryos is determined by the woman's age at the time the eggs were fertilized, not the woman's age at the time the embryos are transferred.