The most highly oxygenated blood in the fetal circulation is found in the

Comprehensive and Detailed Explanation From Exact Extract Sources:

In fetal physiology, the highest oxygen saturation exists in the umbilical vein, which then flows through the ductus venosus before entering the right atrium.

According to Creasy & Resnik Maternal-Fetal Medicine, and AWHONN physiologic foundations:

The umbilical vein carries oxygen-rich blood from the placenta (approx. 80% saturation).

Most of this blood bypasses the liver via the ductus venosus, which therefore contains the most highly oxygenated blood within the fetal circulatory system.

By contrast:

The descending aorta contains mixed blood with significantly lower oxygen content due to mixing after passage through the ductus arteriosus.

The pulmonary arteries in the fetus carry predominantly deoxygenated blood, since fetal lungs are fluid-filled and have high pulmonary vascular resistance.

Thus, the structure containing the highest fetal oxygen concentration is the ductus venosus.