In the fetus, blood entering the right side of the heart has already been oxygenated via the placenta. Because the lungs are not ventilated, only a small amount of blood needs to go through the pulmonary artery. Most of the blood from the right side of the heart bypasses the lungs through the 

• Foramen ovale

• Ductus arteriosus

Normally, these two structures close shortly after birth.