Sperm freezing and embryo freezing (cryopreservation) have been around for decades and are routinely used with assisted reproductive technology. Egg freezing, however, had not been very successful.
A female egg (oocyte) is a large cell that contains quite a bit of water. The older, controlled-rate, slow-freezing technique for eggs caused ice crystals to form in the cell, which could destroy the egg. This is the technique that was primarily used prior to 2002, and success rates were extremely low.
Today the preferred process for freezing eggs is called “vitrification.” Vitrification is, in essence, “flash freezing.” The eggs are placed in a special solution that helps protect them during the process, and the cooling rate is so rapid that ice does not have a chance to form. The mixture of cryoprotectant and egg forms a “glass-like” gel.
Once thawed, eggs that have been vitrified yield much better pregnancy and live birth rates than other options for egg freezing, which is why many fertility clinics now offer this option for women in order to preserve their fertility.