Blood flow through the heart in a fetus differs from that in children and adults. In children and adults, blood picks up oxygen in the lungs. The fetus is not exposed to air. It is inside the uterus, and its lungs are collapsed and filled with amniotic fluid. Because the fetus does not breathe air, the fetus' blood gets oxygen that passes from the mother's blood vessels to the placenta. The oxygen-rich fetal blood in the placenta passes through the umbilical blood vessels (in the umbilical cord) and enters the fetal heart. Only a small amount of blood goes through the lungs. The rest of the blood bypasses the lungs by flowing through two short-cuts (shunts):
The foramen ovale, a hole between the right and left atria
The ductus arteriosus, a blood vessel that connects the pulmonary artery and the aorta
Normally, these two short-cuts close soon after birth.