The primary source of oxygen for the fetus is:

The primary source of oxygen for the fetus is placental transfer, where oxygenated maternal blood delivers oxygen across the placental membrane to fetal blood. The fetal lungs are fluid-filled and non-functional for gas exchange before birth, making them ineffective in oxygen uptake. Oxygenated blood reaches the fetus via the umbilical vein, not the arteries, which carry deoxygenated blood away from the fetus. Amniotic fluid does not play a role in oxygen supply. Therefore, the placenta is essential for fetal oxygenation until the lungs assume this role at birth.

Reference: Williams Obstetrics, Cunningham F. et al., 25th Edition.