The blood gets oxygenated in the placenta and this passes to the umbilical vein. From the umbilical vein, 50% of the blood enters the hepatic circulation and 50% bypasses the liver to enter the inferior vena cava. Thus, the inferior vena cava, which also carries blood from hepatic veins, kidneys, and from lower extremities carries a mixture of deoxygenated and oxygenated blood.
So, the highest percentage of oxygen in fetal circulation is in the umbilical vein, followed by the inferior vena cava. It is only after passing through the inferior vena cava does it go to the aorta.