Why does the ductus arteriosus close off at birth?

The ductus arteriosus closes off after birth so that blood can flow properly through the heart and the lungs, according to the Cleveland Clinic. While in utero, a baby's ductus arteriosus connects the pulmonary artery to the aorta so that blood is shunted from the developing lungs of the fetus.

If the ductus arteriosus doesn't close off properly, blood flows in the opposite direction through the heart and lungs, the Cleveland Clinic states. The extra blood flow causes build-up of blood in the lungs, and the baby may have difficulty breathing. Approximately 10 percent of newborn babies are born with their ductus arteriosus still open, Cleveland Clinic states.