Circulation, fetal: The blood circulation in the fetus (the unborn baby). Before birth, the blood from the heart that is destined (in the pulmonary artery) for the lungs is shunted away from the lungs and returned to the greatest of arteries (the aorta). The shunt is through a short vessel called the ductus arteriosus. When this shunt is open, it is said to be a patent (pronounced pá tent) ductus arteriosus (PDA). The PDA usually closes at or shortly after birth and blood is permitted to course freely to the lungs.