ASRM GUIDELINE 2013
- Evidence indicates that oocyte vitrification and warming should no longer be considered experimentalHuman oocyte cryopreservation (egg freezing) is a process in which a woman’s eggs (oocytes) are extracted, frozen and stored. Later, when she is ready to become pregnant, the eggs can be thawed, fertilized, and transferred to the uterus as embryos.
Oocyte cryopreservation is aimed at three particular groups of women:
Women with cancer who have not yet begun chemotherapy or radiotherapy (Chemotherapy and radiotherapy are toxic for oocytes, leaving few, if any, viable eggs. Egg freezing offers women with cancer the chance to preserve their eggs so that they can have children in the future.)
Those who would like to preserve their future ability to have children, either because they do not yet have a partner, or for other personal or medical reasons (e.g.: Late marriage or having a family history of Premature ovarian failure/Early menopause <40 yrs.)
Egg freezing can also be beneficial for women who, for the purpose of education, career or other reasons, desire to postpone childbearing. Freezing eggs at an early age may ensure a chance for a future pregnancy.
The egg retrieval process for oocyte cryopreservation is the same as that for in vitro fertilization. This includes hormone injections that stimulate ovaries to ripen multiple eggs. When the eggs are mature, final maturation induction is performed, preferably by human chorionic gonadotropin (hCG). The eggs are subsequently removed from the body by transvaginal oocyte retrieval. The procedure is usually conducted under sedation. The eggs are immediately frozen.
The egg is the largest cell in the human body and contains a high amount of water. When the egg is frozen, the ice crystals that form can destroy the integrity of the cell. To prevent this, the egg must be dehydrated prior to freezing. This is done using cryoprotectants which replace most of the water within the cell and inhibit the formation of ice crystals.
Eggs (oocytes) are frozen using the newer flash-freezing process known as vitrification. Vitrification is much faster but requires higher concentrations of cryoprotectants to be added. The result of vitrification is a solid glass-like cell, free of ice crystals.
Vitrification is associated with higher survival rates and better development compared to slow-cooling when applied to oocytes in metaphase II (MII).
During the freezing process, the zona pellucida, or shell of the egg can be modified preventing fertilization. Thus, currently, when eggs are thawed, a special fertilization procedure is performed by an embryologist called ICSI (Intracytoplasmic Sperm Injection) and is also used in IVF.