The umbilical cord has one vein, which carries oxygenated blood from the placenta to your baby, and two arteries that take deoxygenated blood from your baby back to the placenta. A single umbilical artery means that there are only one instead of two arteries carrying deoxygenated blood from your baby. This is the most common umbilical abnormality and occurs in about 1 percent of pregnancies with a single baby. It is much more common with twins or multiples and occurs in about 5 percent of those. This episode answers some questions about the risks involved with this diagnosis.