Embryo freezing allows people to preserve embryos for later use. The biggest challenge of the procedure is the water within embryo cells. While freezing, water might turn into ice crystals that significantly increases the risk of cell damage. 

Cryopreservation ensures prevention of crystal formation. Under this procedure, the water in the cell is replaced with a substance called a cryoprotectant. Therefore, cryopreservation improves the survival rate of frozen embryos.

After water is removed, embryos are cooled to their preservation state. There are 2 methods of freezing:

Slow Freezing: within this method, embryos are placed in sealed tubes and temperature is slowely lowered. Even though slow freezing is effective and reduces the risk of damage, it requires much more time and costs. 

Vitrification: different from slow freezing, in vitrification, embryos are frozen so quickly that the water does not have enough time to form crystals. 

After freezing procedure is complete, embryos are stored in liquid nitrogen at -321° F (-196.1° C)

If stored properly, frozen embryos can survive for decades. In 2020 viable pregnancy resulted 27 years after the embryo was frozen, setting what appears to be a record for the longest-frozen embryo known to have come to birth, according to research staff at the University of Tennessee Preston Medical Library.