An unclosed hole in the main body artery (aorta).

Before a baby is born, the fetus's blood does not need to go to the lungs to get oxygenated.

The ductus arteriosus is a hole that allows the blood to skip the circulation to the lungs.

However, when the baby is born, the blood must receive oxygen in the lungs and this hole is supposed to close.

If the ductus arteriosus is still open (or patent) the blood may skip this necessary step of circulation. The open hole is called the patent ductus arteriosus.