Cryopreservation is the procedure of freezing sperm or fertilized eggs in liquid nitrogen for future reproductive use. In the past decade, the technology behind this procedure has improved and grown to the point where "cryo banks" can be found in nearly every state. A cryo bank for sperm, often called a sperm bank, analyzes, prepares and preserves sperm for extended periods of time, generally one year or up to five years. The sperm is preserved by freezing in liquid nitrogen at 320 degrees below 0 degrees Fahrenheit. Healthy sperm can be obtained from ejaculates or from liquid extracted during a surgical procedure. To date, studies have not indicated a difference or increase in birth defects between sperm that have been through the freeze-thaw procedure and freshly ejaculated sperm. An egg bank or oocyte bank differs from the sperm bank in that it can only freeze fertilized eggs. Unfertilized oocyte preservation is still experimental. Recent studies have indicated problems with damage to chromosomal material. For oocyte cryopreservation, the egg is fertilized by in-vitro fertilization, meaning outside the body in a test tube. It is often a part of the IVF process. The cryopreservation of extra embryos provides another opportunity for embryo transfer at another date. In other words, if the IVF fresh embryo procedure does not result in pregnancy, the frozen embryos can be thawed and transferred to the uterus. The embryo can then be frozen in liquid nitrogen indefinitely, although most banks store the embryos for a maximum of five years. Cryopreservation of human embryos has been successfully conducted worldwide for a number of years. To date, there are no reports of increased birth defects in pregnancies achieved through this process.