Did you know that the umbilical cord has 3 tiny holes? The umbilical cord contains three blood vessels. Two of those are arteries, and one is a vein. The vein carries oxygen and nutrients from the placenta. The placenta is connected to the mother's blood supply. The two arteries transports the waste back to Mom. It is then transported to her kidneys, which filters the waste, and gets rid of it. So, it is safe to say that she's been picking up after her child even during the womb.
Sometimes, one artery is missing, which is more common than you'd think. This condition is called Single Umbilical Artery and it is just that. One artery. There is no concern, just more precautions to take.