circulation

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