PDA is a blood vessel that is present in all babies when they are in the womb. This blood vessel connects the aorta (a large artery that supplies blood from the heart to various parts of the body) and the pulmonary artery (a large artery that supplies blood to the lungs). When the baby is in the womb, PDA normally diverts blood from the pulmonary artery to the aorta as the lungs do not take part in gas exchange.
After birth, when the lungs expand, all the blood in the pulmonary artery flows to the lungs and the PDA which is not needed anymore closes spontaneously in the majority of babies.
However when a baby is born prematurely, this blood vessel may not close as expected, resulting in excessive blood flow to the lungs from the aorta.