One of the three blood vessels, usually one large umbilical vein and two small umbilical arteries, buried within Wharton's jelly, that transport blood to and from the placenta, where exchange between the mother and fetus takes place; the umbilical vein carries oxygenated, nutrient-rich blood from the placenta to the fetus, and the umbilical arteries carry deoxygenated, nutrient-depleted blood from the fetus to the placenta. [ ]