Egg freezing, also known as oocyte cryopreservation, is a medical procedure in which the eggs from a women are extracted, frozen and stored for use at a later time. Typically, this procedure is used to preserve a woman's fertility when undergoing chemotherapy or radiation therapy for cancer. The eggs can be fertilized in vitro, and then implanted into the uterus when desired.
The use of cryopreservation in fertility treatments started with experiments on mice in the late 1960's and early 1970's. The first successful mammalian birth resulting from a frozen mouse embryo took place in 1972. Afterward, the techniques were adapted for human embryos.
The first successful pregnancy with frozen human embryos occurred in 1983. The first successful pregnancy from a frozen egg occurred in 1986. Embryos are fertilized eggs. Freezing embryos and then thawing them for implantation in a surrogate has a higher success rate than egg freezing. When an unfertilized egg is thawed for in vitro fertilization and subsequent implantation, it can undergo premature changes that may prevent a successful pregnancy.
The medium in which eggs are frozen is called the cryoprotectant. It is a substance that prevents ice crystals from forming within the egg during the freezing process. Research into the most effective cryoprotectant has determined that sucrose and propylene glycol are best suited to protect the egg.
Eggs which are frozen slowly and thawed rapidly have the best chance for success. Limiting the exposure to the cryoprotectant during the thawing process, by using progressive dilution, reduces the risk of damage to the egg.