The History of Embryo Freezing

Embryo freezing, or embryo cryopreservation, is a technique used to preserve a fertilized egg. After an egg is fertilized with a sperm via in vitro fertilization, the resulting embryo is frozen and stored for use at a later time. When a woman is ready to have a child, the embryo is thawed and implanted into the uterus.

Initial Freezing Methods

The first embryo was successfully frozen, thawed and implanted in 1972. The process of freezing was slow. The embryo was cooled down to -80 degrees Celsius at a rate of one degree per minute. Then, the embryo was rapidly cooled to -197 degrees Celsius by placing it into liquid nitrogen.

Initial Thawing Methods

During the thawing process, the embryo is placed in a water bath. Gradually, the temperature is raised. As the embryo thaws, the cryoprotectant, which is a preservative used during the freezing process, will leave the embryo. A step wise dilution is required to limit the effects of the cryoprotectant leaving the embryo.


A quicker and more effective method for freezing embryos is vitrification. An embryo is placed directly in liquid nitrogen in order to preserve the embryo's physical state without producing any ice crystals. The process requires a large concentration of cryoprotectant, which can be damaging to the embryo if its exposed to it at room temperature. In addition, the liquid nitrogen itself poses a risk of contamination. Infectious agents can thrive in liquid nitrogen and can infect the frozen embryo.

Embryo Stages

Embryos can be frozen within seven days of fertilization, which is any time between the pronuclear stage and blastocyst stage. Different protocols are required for each stage.  


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