While still in the mother's womb, a baby does not need their lungs to supply oxygen because they receive oxygen from their mother. Since a baby's lungs do not provide oxygen, there is no need for the heart to pump blood to the lungs. The ductus arteriosus is a blood vessel that is present in all babies while still in the womb, and it allows blood to bypass the lungs.

When the baby is born and the umbilical cord is cut, their lungs need to supply oxygen to their body. Their lungs expand, their blood vessels relax to accept more blood flow, and the ductus arteriosus usually closes within the first hours of life. Sometimes, the ductus arteriosus does not close on its own. This is known as a patent ("open") ductus arteriosus. While this condition is seen more often in premature babies, it may also appear in full-term infants.