Cryopreservation is a technique used to freeze and then thaw sperm and embryos for use in vitro fertilization (IVF) cycles. With the availability of frozen embryos, a woman doesn't need to undergo stimulation by fertility drugs in order to have an embryo transfer during an infertility treatment cycle.
During the standard course of infertility treatment, hormones are used to stimulate the development of multiple eggs. After these eggs are retrieved and fertilized in the laboratory, there may be more embryos created than can reasonably be transferred to the woman’s uterus. When these “extra” embryos are of sufficient quality, they may be cryopreserved (frozen) so that they may be transferred at a future date. Embryo cryopreservation, which is possible in approximately 25% of IVF cycles, provides the opportunity to have an additional embryo transfer without the inconvenience and expense of a full IVF cycle.
Embryos may be frozen at any stage between day 1 and day 6 after egg retrieval. However, not all embryos are candidates for cryopreservation. Unfortunately, some may be damaged by cryopreservation and experience has shown that high quality embryos are far more likely than others to survive and be capable of further development after freezing. Embryos that divide slowly, or are irregular in other ways, do not fare well after cryopreservation and, therefore, are not frozen. Because problems such as these are more prevalent in older women, cryopreservation usually is not recommended for patients age 40 and older.
After placing the embryos in a cryoprotectant solution, they are frozen in a computer-controlled device designed specifically for embryo cryopreservation. Long-term embryo storage is in liquid nitrogen, at a temperature of –320oF (-196oC).
Cryopreserved embryos are replaced during either a natural menstrual cycle or a hormonally controlled cycle. Considerable care is taken to minimize the possibility of damage caused by cryopreservation. Depending on the embryo stage at the time of freezing, between 60 and 90 % survive the freeze/thaw process. The pregnancy rate after transfer of these embryos is similar to that of fresh embryos.
Extended periods of storage in liquid nitrogen have no apparent affect on their viability. Embryos thawed after several years of cryostorage fare as well as those frozen for only one or two months. Since 1983, the cryopreservation procedure has resulted in the birth of thousands of babies worldwide. With this extensive experience, there have been no reports of any increase in birth defects as a result of cryopreservation.
While embryos are reliably and routinely cryopreserved, it is not currently possible to efficiently freeze human eggs (oocyte cryopreservation). There have been isolated reports of pregnancies that resulted from frozen eggs, but the few IVF programs attempting this procedure have experienced very low success rates.