Internal Iliac Veins
Pelvic veins are more variable than the arteries, but share similar names.
The internal iliac vein is the main site of drainage of the smaller pelvic veins. It begins superior to the sciatic foramen as the continuation of the superior gluteal vein. It receives the superior/inferior vesical veins, the middle rectal vein, pudendal vein and obturator vein.
The internal iliac veins unite with the external iliac veins anterior to the sacroiliac joints to form the common iliac veins.
External Iliac Veins
The external iliac veins are the continuation of the common femoral veins, beginning beneath the inguinal ligament and medial to the artery. They pass, medial and posterior to the artery, along the superficial surface of the psoas muscle along the pelvic inlet. It receives the same tributaries as the external iliac artery (inferior epigastric vein and deep circumflex iliac vein).
Common Iliac Veins
The common iliac veins begin anterior to the sacroiliac joint, posterior and inferior to the bifurcation of the common iliac arteries. The left is longer than the right, and passes posteriorly to the left and right common iliac arteries. The shorter and more vertical right common iliac passes superiorly. The two vessels unite anterior to the L5 vertebral body to form the inferior vena cava.