OOCYTE CRYOPRESERVATION
Egg freezing is a relatively new and developing technology. The first baby born using a frozen egg was in 1986. However, high failure rates and concerns that freezing would damage the egg chromosomes and cause birth defects, has kept egg freezing on the sidelines for over a decade.
Who would benefit from egg freezing?
Several groups may potentially benefit from egg freezing:
- Preservation of fertility before a young single cancer patient receives radiotherapy or chemotherapy.
- Women who need to have their ovaries removed such as women who are carriers of BRC1 and II mutation genes in order to reduce the risk of developing breast cancer.
- As a way to salvage an IVF cycle when the partner is unable to produce a semen sample on the day of egg collection.
- Alternative to embryo freezing especially in countries where embryo freezing is outlawed.
- For couples for whom embryo freezing presents a moral or ethical dilemma.
- Women in their early 30s and career oriented women thinking of delaying motherhood. Although there are no guarantees, a woman is more likely to get pregnant at 40 with one of her eggs frozen in her early 30s than with her 40-year-old fresh eggs.
- Women with a family history of premature ovarian failure wishing to preserve their fertility. Some women wish to freeze eggs for their daughters who have recently been diagnosed as having Turner’s syndrome.
How old is too old to freeze eggs for social reasons?
Women who freeze their eggs when they are under 30 years have higher rates of success compared with women who are older. If possible women seeking elective freezing of their eggs should do so when they are young in order to increase their likelihood of achieving their fertility goals.
How are the eggs frozen?
Egg freezing entails a costly and invasive IVF treatment. After the eggs are collected they are frozen. The human egg is the largest cell in the human body. It contains huge amounts of water and its outer layer is fairly water-resistant. Problems occur during freezing as this water will turn to ice crystals, which disrupt the egg’s DNA and membrane structure. Therefore cryoprotectants (antifreeze solutions) are used to replace this water to reduce ice formation. Freezing eggs should be performed within 2 hours of egg collection. There are two techniques for freezing eggs:
- The traditional “slow controlled freeze” method where the eggs are frozen slowly using cryoprotectants, placed in a vial or straw and cooled gradually. At -32° C the straw is put into liquid nitrogen (-196° C) this technique results in one baby per 100 eggs.
- Vitrification “ultrarapid freezing”. This is a new method where the vial or straw is plunged directly into liquid nitrogen, the cooling rates are so rapid that ice crystals does not have a chance to form, and the mixture of cryoprotectant and egg forms a “glass-like” solidification of cells. Vitrification method appears to be associated with a significantly higher survival rates compared to slow freezing.
The usual period for storing eggs in the UK is 10 years, however it is possible to store the eggs for longer if the woman was under 45 when the eggs were collected. Regardless of the method of freezing used the outer coat of the egg (zona pelluicida) becomes hardened. Hence, after thawing the eggs, they would then have to be fertilized by ICSI.
How successful is egg freezing?
Antinori and co-workers from Italy (ESHRE 2007) reported very high rates of egg survival (99%), fertilization rates (92%), pregnancy rates (32%) and healthy babies using virtification technique. Based on current data, egg freezing appears to be safe. Fortunately, to date no abnormalities have been reported from the 200 babies born worldwide.
