<P> In placental mammals, the umbilical cord (also called the navel string, birth cord or funiculus umbilicalis) is a conduit between the developing embryo or fetus and the placenta . During prenatal development, the umbilical cord is physiologically and genetically part of the fetus and (in humans) normally contains two arteries (the umbilical arteries) and one vein (the umbilical vein), buried within Wharton's jelly . The umbilical vein supplies the fetus with oxygenated, nutrient - rich blood from the placenta . Conversely, the fetal heart pumps deoxygenated, nutrient - depleted blood through the umbilical arteries back to the placenta . </P> <P> The umbilical cord develops from and contains remnants of the yolk sac and allantois . It forms by the fifth week of development, replacing the yolk sac as the source of nutrients for the embryo . The cord is not directly connected to the mother's circulatory system, but instead joins the placenta, which transfers materials to and from the maternal blood without allowing direct mixing . The length of the umbilical cord is approximately equal to the crown - rump length of the fetus throughout pregnancy . The umbilical cord in a full term neonate is usually about 50 centimeters (20 in) long and about 2 centimeters (0.75 in) in diameter . This diameter decreases rapidly within the placenta . The fully patent umbilical artery has two main layers: an outer layer consisting of circularly arranged smooth muscle cells and an inner layer which shows rather irregularly and loosely arranged cells embedded in abundant ground substance staining metachromatic . The smooth muscle cells of the layer are rather poorly differentiated, contain only a few tiny myofilaments and are thereby unlikely to contribute actively to the process of post-natal closure . </P> <P> The umbilical cord contains Wharton's jelly, a gelatinous substance made largely from mucopolysaccharides which protects the blood vessels inside . It contains one vein, which carries oxygenated, nutrient - rich blood to the fetus, and two arteries that carry deoxygenated, nutrient - depleted blood away . Occasionally, only two vessels (one vein and one artery) are present in the umbilical cord . This is sometimes related to fetal abnormalities, but it may also occur without accompanying problems . </P> <P> It is unusual for a vein to carry oxygenated blood and for arteries to carry deoxygenated blood (the only other examples being the pulmonary veins and arteries, connecting the lungs to the heart). However, this naming convention reflects the fact that the umbilical vein carries blood towards the fetus's heart, while the umbilical arteries carry blood away . </P>

How many vessels are in the umbilical cord
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