During the process of in vitro fertilization (IVF) multiple embryos (fertilized eggs) are often created. After the desired number of embryos is implanted in the uterus, there may be embryos left over and the decision may be made to have them cryopreserved (frozen) for use at a later date. If there have been previous infertility problems, or if the first implantation procedure is not successful, one or more of these embryos may be used in the future. Cryopreservation helps to make any future implantation process simpler, less expensive and less invasive than the initial process, because ovarian stimulation and egg retrieval will be unnecessary. Cryopreservation can also be used to preserve embryos prior to cancer treatments.
Embryo Cryopreservation Methods
There are two methods used to cryopreserve embryos: conventional (slow freezing) and vitrification (fast freezing). In the conventional freezing method, embryos are slowly frozen to 196 degrees Celsius. During vitrification, embryos undergo a process of rapid freezing that eliminates the potentially damaging formation of ice crystals that can occur with the conventional method. Both methods of cryopreserving embryos are similar in that they allow for preservation and storage of embryos in liquid nitrogen at very cold temperatures, and both use substances known as cryoprotectants to protect embryonic cells from damage during freezing, storage and thawing.
Results of Embryo Cryopreservation
Once frozen, embryos can be stored for prolonged periods. It is possible, however, that some embryos will not survive freezing or thawing. Although many successful live births have resulted from using frozen embryos, they are less likely to lead successful pregnancies than fresh embryos.