Embryo cryopreservation is a method used to preserve embryos by cooling and storing them at low temperatures. They can be thawed at a future date and transferred to the uterus in an unstimulated cycle.
The benefit of embryo cryopreservation is that it permits the use of thawed embryos in an otherwise natural cycle, sparing the patient from undergoing ovulation induction, egg retrieval and the associated costs.
Despite the potential beneficial outcome of human embryo cryopreservation, there is always a risk that some or all embryos will not survive the freezing and thawing procedure. Approximately 50% of all cryopreserved embryos will not survive.
There is no increased risk of an abnormality from cryopreservation reported in the general population.
Cryopreservation techniques involve procedures which are critical to survival of the embryo. One of the most important principles of this technique is to remove water from the cells before they freeze. If the water is not removed, large intracellular ice crystals are formed during freezing which may damage the embryo. In order to remove water from the embryos, special solutions called cryoprotectants are used. The embryos are briefly exposed to increased concentrations of cryoprotectants. They are then placed inside plastic straws (filled with one of the cryoprotectants) and transferred to a controlled-rate freezer where the embryos are cooled slowly. Once the temperature of the embryos has reached a predetermined point below 0°C, the straws are plunged into liquid nitrogen (-196°C) and stored there until thawing.
To thaw the embryos, the straws are pulled out of the storage tank and warmed first at room temperature and then in a water bath at 37°C. The embryos are released from the straws and rinsed in decreasing concentration of cryoprotectants to replace cellular water. They are then placed in a culture medium and are incubated for 1-2 hours to determine if they have survived the freeze/thaw process.