In contrast to the right common iliac vein, which ascends almost vertically to the inferior vena cava, the left common iliac vein takes a more transverse course. Along this course, it underlies the right common iliac artery, which may compress it against the lumbar spine.
References
^ May R, Thurner J (1957). "The cause of the predominantly sinistral occurrence of thrombosis of the pelvic veins". Angiology8 (5): 419–27. PMID 13478912.
^ Fazel R, Froehlich JB, Williams DM, Saint S, Nallamothu BK (2007). "Clinical problem-solving. A sinister development--a 35-year-old woman presented to the emergency department with a 2-day history of progressive swelling and pain in her left leg, without antecedent trauma". N. Engl. J. Med.357 (1): 53–9. doi:10.1056/NEJMcps061337. PMID 17611208.