Anterior view of fetus in utero showing basic anatomy of the lower great vessels with emphasis on the bifurcation of the aorta and the femoral veins. There is further compression of the iliac veins by the gravid uterus and the location of the right iliac artery over the left iliac vein. Hormonally mediated vein dilation is also shown. 78 to 90% of Venous Thromboembolisms occur in the left ilio-femoral vein.