In the fetus, after the heart is developed, the chambers are the same as in the normal heart with a few differences. Since the lungs have not yet opened in the fetus, very little blood flows to the lungs. Blood also flows to the mother's placenta which supplies oxygen for the blood. Because of these differences, a number of structures are present in the fetus disappear after birth.
One is a ductus arterious, a blood vessel or duct between the aorta and pulmonary artery. Since it is open or patent, it is called a patent ductus arteriosus or PDA. Normally, this vessel closes within hours to days after birth. Another structure is an opening between the right and left atrium called a foramen ovale meaning “oval hole.” Since this hole is open or patent, it is called a patent foramen ovale or PFO. See the non-anatomical illustration of fetal flow.
With these structures, the blood flow in the fetus is different than in babies or children. Blood from the placenta (carrying oxygen) mixes with blood from the veins of the upper and lower half of the body (the superior and inferior vena cava or SVC and IVC for short), and enters the right atrium. Some of the blood (mostly the blood with more oxygen from the placenta) is directed to the left atrium through the PFO.
The rest of the blood in the right atrium goes to the right ventricle. Usually, this blood has less oxygen in it. The blood then gets pumped from the right ventricle out to the pulmonary arteries, but the lungs are not open and therefore there is too much resistance for the blood to go to the lungs. Instead, the blood passes through the PDA and goes to the aorta.
From the there, it goes to the placenta where it receives oxygen. On the other hand, blood that passed into the left atrium (which has more oxygen) goes to the left ventricle where it is then pumped to the aorta. Most of this blood in the aorta goes to the upper half of the body (the brain), and only a smaller percentage goes to the lower half of the body. Of course, in the fetus, the lower half of the body gets blood that crosses the PDA as mentioned above.
Learn more about fetal cardiology and The Chicago Institute for Fetal Health.