Cryopreservation of embryos has always been an important tool in an IVF program. It enables a precautious policy for embryo transfer, lessening the chance of multiple pregnancies, knowing that the embryos not transferred will be available for subsequent thawed cycles. This also establishes a cumulative pregnancy rate, increasing the overall chances for patients to conceive per IVF cycle, as well as helping patient management with complications such as Ovarian Hyperstimulation Syndrome (OHSS). However, results from frozen/thawed cycles are often disappointing with success rates usually around half that of a fresh cycle.
Embryos are placed in a series of solutions to draw some of the water out by osmosis and then add a cryoprotectant. The removal of water helps prevent the formation of damaging ice crystals and the cryoprotectant protects the embryos during the freezing process.
The embryo freezing process takes approximately 3 hours. The temperature is slowly decreased to -36?C and then plunged into and stored in liquid nitrogen at -196?C for long-term storage.
Embryo thawing is the reverse of the freezing process, and involves warming the embryos to room temperature to allow the transfer back into culture media at 37?C in an incubator. The embryos are then ready for transfer to the uterus. The embryos are thawed either the day before or on the day of the scheduled embryo transfer. Embryo thawing takes approximately 2 hours. Sometimes individual cells within the embryo are damaged by the freezing process. Embryos with some freeze damage can still go on to produce a healthy pregnancy however the more the embryo is damaged the less likely it will be for the embryo to develop. Sometimes all of the cells within the embryo are damaged. In this situation the embryo will not be transferred.