When the fetus is in utero which means it is inside the uterus, the primary oxygen delivery and and carbon dioxide excretion mechanism works through the placenta. The fetus is submerged in amniotic fluid and the lung is still maturing.
Placenta is an organ that belongs to the fetus which is bound to the endometrium (the innermost wall of the uterus), it is involved not only in air exchange but also in providing nutrients and antibodies and excretion of other metabolic waste products. It should be noted that there is absolutely no mixing of fetal and maternal blood happening in utero.
During this time the veins and arteries to and from the lungs are shut down. Oxygenated blood coming from the placenta are transported to the inferior vena cava (IVC) of the fetus via the umbilical vein. This happens via a vessel called ductus venouses.
Once the oxygenated blood enters the IVC it is moved parallel with the deoxygenated blood coming from the fetal organs. This mixed blood enter into the right atrium.
Fetal heart is different from its adult counterpart. Fetal heart has foremen ovale which is a defect in the atrial part of the atrioventricular septum. This allows blood to flow from the right atrium to the left atrium, then to the left ventricle, where it is pumped into the aorta, into the systemic circulation. Like in an adult. The shunting of oxygenated blood from right to left atrium will supply blood high in oxygen and nutrient content to the upper extremities, including the critically important brain.
However, the fate of the blood that does not cross over into the left atrium is drastically different. They would enter the right ventricle and then get pumped into the pulmonary artery, which is where the deoxygenated blood goes to get oxygenated from the lung. However, since the fetal lungs are collapsed, that path is blocked. To enter the systemic circulation there is a bypass tract called ductus arteriosus which connects the pulmonary trunk with the aorta. This is the path the blood takes into the systemic circulation of the fetus.
Some of the blood moves from the aorta through the internal iliac arteries to the umbilical arteries, and re-enters the placenta, where carbon dioxide and other waste products from the fetus are taken up and enter the maternal circulation.
Fetal circulation may look counterintuitive at first. It is and and also feels less efficient than adult circulation. Specially the part where oxygenated and deoxygenated blood getting mixed together in the IVC. However, it should also be noted that this is a temporary measure “duct-taped” together to make the system function until birth.
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