As seen in the diagram above, the left common iliac vein is predisposed to be compressed by the right common iliac artery. This can lead to stasis and eventually causing thrombosis. Another effect is that the pulsatile nature of the artery over the vein leads to turbulence in the blood flow, thereby favouring thrombosis.
Because of this anatomical predisposition, most Deep Vein Thrombosis (DVT) seen during pregnancy occur in the left iliac vein system. All the classic investigations done for cases of DVT should be performed here also.