Egg freezing, also known as mature oocyte cryopreservation, is a method used to save women's ability to get pregnant in the future. Eggs retrieved from the woman's ovaries are frozen unfertilized and stored for later use. However, success in live birth rates in older age women and women undergoing cancer treatment may be lower compared to success rates achieved by frozen and thawed embryos and sperm cells.  Older women have higher miscarriage rates, mainly due to having poor-quality and older eggs. Certain medical treatments such as radiation or chemotherapy may also cause a reduction in the number of eggs and egg quality. For example, while the risk of a 25-30 years old woman giving birth to a baby with Down syndrome is 1 out of 1000-1200 pregnancies, this rate drops to 1 out of 100 pregnancies at the age of 40. The low number of eggs and poor egg quality due to older age and/or cancer therapy are the main factors affecting live birth rates with frozen-thawed eggs.

Oocyte cryopreservation has become very popular today for social reasons. Economic conditions all over the world have led women to have children at an advanced age. Especially in countries like Turkey, where marriage is required to create embryos and donation programs are prohibited, the only alternative for single women to preserve their fertility is oocyte freezing.In addition, in countries where the cryopreserved embryos of the couple should also be destroyed in case of termination of marriage, it seems that oocyte cryopreservation should be performed in addition to the embryo cryopreservation in order to have children in the future, especially in cancer patients.

Today, vitrification is the most commonly used process for egg freezing. High concentrations of substances that help prevent ice crystals from forming during the freezing process (cryoprotectants) are used with rapid cooling. In a single instant, the cells pass from ambient temperature to the temperature of liquid nitrogen, which is -196°C, and instantaneously solidify into a glass-like structure.  Thus, the integrity of the cells is preserved without being damaged.The post-thaw survival rate in the vitrified eggs is significantly high and there is no problem other than hardening only in the zonapellucida, which surrounds the egg cell.Therefore, intracytoplasmic sperm insemination (ICSI) is recommended to create embryos with these cells.However, the success rates especially oocyte donation for women over 36 years of age or for cancer patients, have remained low despite everything. Therefore, the optimal number of oocytes that should be retrieved in each age range to have a chance of pregnancy in the future has been determined.