Embryo Cryopreservation (Freezing): Embryo cryopreservation (the process of freezing, storage and thawing embryos) can enhance pregnancy rates by allowing excess embryos not replaced in a fresh embryo transfer to be stored for future use.
Embryo freezing may also be performed when a fresh embryo transfer is not performed for any of the following reasons: (1) Risk of ovarian hyperstimulation syndrome (OHSS), (2) Poor quality endometrium (a thin uterine lining), (3) Intermenstrual bleeding, (4) Planned "banking" cycle in which the patient elects to store all embryos, (5) Extremely difficult embryo transfer.
Embryos are placed into straws or vials containing anti-freeze or cryoprotectant solutions. These are transferred to a programmable biological freezer which is used to achieve a controlled slow rate of cooling. During cooling, cells dehydrate and as the temperature is reduced, more ice forms and water is removed gradually from the cells. Slow cooling is continued to ~ -35°C at which point embryos are rapidly cooled by plunging into liquid nitrogen (-196°C). Embryos are kept in storage tanks of liquid nitrogen until thawing is performed.
This definition is from fertile.com