umbilical cord (ŭmbĭlˈĭkəl) [key], cordlike structure about 22 in. (56 cm) long in the pregnant human female, extending from the abdominal wall of the fetus to the placenta. Its chief function is to carry nourishment and oxygen from the placenta to the fetus and return waste products to the placenta from the fetus. It consists of a continuation of the membrane covering the fetus and encloses a mucoid jelly through which one vein carries oxygenated blood and two arteries carry unoxygenated blood. After birth, the cord is clamped off and cut. It is sometimes abnormal in length and may break prematurely or form loops or knots, which may asphyxiate the fetus. The stump of the cord that is left attached to the infant withers and drops off, leaving the scar known as the navel.
Because umbilical cord blood is especially rich in stem cells (cells that give rise to red blood cells and lymphocytes) some parents choose to save it in private cord blood banks in case of future need as a transplant alternative to bone marrow, but in many diseases treated with stem cells such autologous transplants are contraindicated. Studies have shown that people not related to the donor (genetically mismatched) can benefit from transplants of umbilical cord blood in combating leukemia and other cancers. Cord blood has also been used to repair heart and other tissue defects in children with certain metabolic disorders.