At this point, the ductus arteriosus is not needed to bypass the lungs. Under normal circumstances, within the first few days or weeks after birth, the ductus arteriosus closes and blood no longer passes through it. Most babies have a closed ductus arteriosus by 72 hours after birth. In some babies, however, the ductus arteriosus remains open (patent) and becomes a problem, PDA. The opening between the aorta and the pulmonary artery allows oxygen-rich (red) blood to pass back through the blood vessels in the lungs.