The fetal circulation is designed to serve prenatal needs. Very little blood goes to the lungs before birth because they are nonfunctional.
The well-oxygenated blood returns from the placenta into the umbilical vein, goes through the ductus venosus and vena cava inferior (which also receives deoxygenated blood from the portal vein) to the right atrium. Most of the blood goes through the foramen ovale into the left atrium and mixes with the blood from the pulmonary veins. The blood passes into the left ventricle and leaves via the ascending aorta.
A small amount of blood goes from the right atrium into the right ventricle, leaves by the pulmonary trunk and through the ductus arteriosus into the aorta. The mixed blood from the descending aorta returns to the placenta through umbilical arteries.
At birth, the circulation of the fetal blood through the placenta ceases and the lungs begin to function.
The foramen ovale, ductus arteriosus, ductus venosus and umbilical vessels subsequently close or transform into corresponding ligaments.
