Fetal circulation: 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.
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