In case a "fresh" IVF cycle is not successful, cryopreservation of supernumerary embryos allows for a new embryo transfer and possible pregnancy without having to submit the patient again to the ovarian stimulation and egg collection procedures. The success rate of this procedure is slightly lower than that of the "fresh" cycle. Human embryos have a high water content and are therefore highly sensitive to freezing. In either case (cleavage stage embryos or blastocysts), the freezing process is the same and includes placing in special cryoprotectant solutions and storing in liquid nitrogen in separate thin straws, bearing the names of the couple members.
There are two cryopreservation techniques, one being the controlled-rate slow freezing and the other being ultra-rapid freezing (vitrification). In the slow freezing method, the cooling rate is approximately -0.3°C/min, whereas in ultra-rapid freezing this rate is over -20,000°C/min. Vitrification is not a new technique; however, it has gained universal acceptance after 2010, because its advantages. Vitrification avoids destruction caused by ice crystal formation within the cells and significantly increases the survival rate of embryos upon thawing. It is also possible to closely monitor and fully control the procedure in the lab. Safety of cryopreservation methods has now been fully documented in the international literature. Studies show that children born after cryopreservation are perfectly healthy, even if the embryos have been kept in cryostorage for several years (up to 5).