As in an adult heart, the prenatal heart develops four chambers and four valves. But because the fetal lungs will not be used until after birth, blood must bypass the lungs. Two structures develop in the prenatal heart that allow the blood to be routed around the lungs: the foramen ovale and the ductus arteriosus. The foramen ovale is a hole that exists between the left and right atria. The ductus arteriosus is a blood vessel that connects the aorta to the pulmonary artery.