There are several techniques for embryo cryopreservation, and research is ongoing. Traditional methods include “slow,” graduated freezing in a computerized setting, and “rapid” freezing methods, called “vitrification.” Most of the data on the success of frozen embryo transfer reported in the medical literature are derived from embryos frozen with the traditional slow freeze method. Recent studies comparing vitrification versus traditional method indicate that vitrification is at least as good as, if not superior to, traditional slow freeze method.
Embryos can be frozen at different stages: pronuclear stage, cleavage stage or blastocyst stage. Depending on the stage at which embryos are frozen, freezing of the embryos can be performed anywhere from the first to the sixth day after fertilization. Generally only normally- developing embryos will be frozen. The embryos will be transferred through a series of solutions with increasing concentration of several cryoprotectants, substances known to protect embryos against damages from freezing, and then frozen. After the embryos are frozen, they are stored at a very cold temperature in liquid nitrogen (-196 °C; -321 °F). After a frozen embryo is thawed for intrauterine transfer, it is treated in a manner similar to that used in the IVF laboratory for non-frozen fresh embryos. Any thawed embryo which does not appear to be viable will not be transferred.