Following IVF treatment, there may be more embryos created than can reasonably be transferred to the woman’s uterus. In approximately 25% of IVF cycles the remaining embryos are of sufficient quality to be frozen and stored for a future use, giving couples the opportunity to have an additional embryo transfer without the expense of a full IVF cycle.
In all cases the embryo freezing procedure involves placing the embryos in a cryoprotectant solution and cooling them under controlled conditions. Then the embryos are stored, for up to five years, in liquid nitrogen at low temperature (-196oC). Considerable care is taken to minimize the possibility of damage caused by the whole procedure.
Embryos can be frozen at any stage between day 1 (2PN stage) and day 6 (blastocyst stage) after egg retrieval. Only high-quality embryos are cryopreserved because are far more likely than others to survive and be capable of further development after freezing. Depending on the embryo stage at the time of freezing, between 60-90% survive freeze/thaw process resulting to a future pregnancy. Cryopreserved embryos are placed in the uterus during either a natural menstrual cycle or a hormonally controlled cycle.