Cryopreservation has been around for decades. Scientists have been freezing sperm since the 1950s, and they have been freezing embryos since the 1980s.
Human eggs, however, have a high water content. If conventional, slow-freezing techniques are used to freeze eggs, the high water content makes the egg susceptible to ice crystal formation. Upon thawing the egg, the ice crystals can fracture and damage it.
In recent years, the egg freezing method that has come into favor is vitrification. Vitrification is a rapid freezing technique that does not allow ice crystals to form in the egg. The technique involves newer cryoprotectants, substances that protect biological tissue from freezing damage.
The egg is placed in a bath with a cryoprotectant along with sucrose (sugar) to help draw some of the water out of the egg. Then, the egg is placed in a high concentration of cryoprotectant for less than a minute and quickly placed in a liquid nitrogen bath that instantaneously freezes the egg.
The rapid cooling rates keep the eggs from forming ice crystals, which means the egg is more likely to be thawed successfully. By being able to freeze and thaw eggs successfully, this has significantly improved pregnancy success rates from frozen eggs, thus opening up the ability for women to preserve their fertility for the future regardless of whether they have a partner or not.