The inferior vena cava (IVC) is formed by the confluence of the right and left common iliac veins at the level of approximately L5. The hepatic veins drain into the IVC superiorly but do not form it. The superior mesenteric and splenic veins join to form the portal vein, not the IVC.

According to Moore’s Clinically Oriented Anatomy:

“The IVC begins at the level of L5 by the union of the right and left common iliac veins.”