The Umbilical Cord usually has three vessels, two arteries that carry waste from the baby back through the placenta to the mother and one vein that carries nutrients and oxygen to the baby from the mother via the placenta.
I teach my students to remember it with the smilie face, Two a's and a V. a a
About 1.5% of pregnancies result in a single artery cord. This is a cord with one artery and one vein. In most cases it won't affect the baby, though in a few there is a chance of some complications.