Cryopreservation, thawing, and transfer of human embryos

Cryopreservation of human embryos is a routine procedure in most assisted reproductive technology laboratories. Embryos can be successfully frozen at the pronuclear, multicellular (four to eight blastomeres), and blastocyst stages. Each laboratory has to determine which freezing protocol is best suited for its patients. Freezing protocols have been developed that utilize cryoprotective agents such as glycerol, propanediol, dimethylsulfoxide, and sucrose. The purpose of these cryoprotectants is to help reduce the formation of intracellular ice when using a slow cooling protocol to freeze the embryos. The two most common ways to freeze embryos utilize either low concentrations of cryoprotectants and a slow stepwise freeze or high concentrations of cryoprotectants and a rapid freeze. Frozen-thawed embryos can be transferred to naturally cycling women or to women who have been primed with hormone replacement treatment with equal success.