Before a baby is born, blood flow normally bypasses its lungs because oxygen and nutrients are received from the placenta and the umbilical cord. After birth, when the infant begins to breathe, two structures in the heart -- the foramen ovale and the ductus arteriosus -- close in order to deliver fully oxygenated blood to the body.

If the ducts do not close at birth, some of the blood in the left side of the heart will go to the lungs instead of into general circulation. The disorder can occur in both premature and full-term infants. Usually symptoms are mild, but they become more pronounced if not corrected by the age of 2. Risk factors are prematurity and respiratory distress syndrome. The incidence is 1 in 2500-5000 infants.