The lifeline of the fetus during its stage of intrauterine development, the umbilical cord averages 50 to 60 centimeters (20 to 23 inches) in length in a full term pregnancy and connects the fetus to the placenta. Contained within the cord are one umbilical vein, which transfers from the placenta the oxygen and nutrients necessary for fetal growth and development, and two umbilical arteries, which return the carbon dioxide and metabolic waste products produced by the fetus back to the placenta for elimination by the mother. These blood vessels are wrapped within a protective spongy material called Wharton's jelly.
Umbilical cords are often coiled, an arrangement that is thought to protect the blood vessels from the external compressive forces of uterine contractions. Some infants are born with the umbilical cord wrapped around the neck or a body part. Rarely, fetal movements can actually tie a knot in the cord. For the most part, fetuses can tolerate these stresses well and do not end up with major problems.
Creasy, Robert K., and Robert Resnik. Maternal-Fetal Medicine. Philadelphia: Saunders, 1999.
Gabbe, Steven, Jennifer R. Neibyl, and Joseph L. Simpson. Obstetrics: Normal and Problem Pregnancies. New York: Churchill Livingstone, 1997.