In placental mammals, the umbilical cord is a tube that connects a developing embryo or fetus to the placenta. It normally contains three vessels, two arteries and one vein, buried within Wharton's jelly, for the exchange of nutrient- and oxygen-rich blood between the embryo and placenta. The presence of only two vessels in the cord is sometimes related to abnormalities in the fetus, but may occur without accompanying abnormalities.