The umbilical cord has one vein, which is carrying 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 instead of two arteries carrying deoxygenated blood from your baby there is only one. 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.