Freezing

Cryopreservation, or freezing, is a tool that allows for the preservation of sperm, eggs, and embryos. Freezing sperm has been possible for decades, and embryo freezing is now routine. Egg freezing is performed more rarely.

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The main danger in freezing delicate eggs, sperm, and embryos is that ice crystals will form and destroy the cells. To avoid that, materials called cryoprotectants are added to the mixture holding the cells to protect them during freezing. Still, most cryopreserved specimens are not quite as viable as fresh specimens.

After embryos are transferred to the uterus for in vitro fertilization, any excess normal-looking embryos may be frozen for later IVF cycles. You must pay to have the embryos frozen, stored, and thawed later, but this is generally cheaper--and less trouble--than going through ovulation induction and egg retrieval again. Storing embryos usually costs a few hundred dollars per year.

There are few circumstances in which eggs need to be frozen. Most of the women who choose to freeze their eggs are patients with cancer or other diseases who know their ovaries will be affected by treatment. Some women decide to freeze eggs when they are in their 30s and aren't sure when they will be ready to have children. Eggs that have been frozen must be fertilized using intracytoplasmic sperm injection (ICSI), in which a single sperm is injected into an egg.

Researchers are constantly working on better cryopreservation techniques that improve the chances that embryos and eggs will survive freezing and be viable after they are thawed.