Compression (squashing) of the pelvic veins can be due to a number of causes. The most common cause is “May Thurner Syndrome” (MTS).
May Thurner Syndrome describes a condition where the left (and sometimes the right) common iliac vein is compressed (squashed) between the right common iliac artery and the spine. It is an anatomical variant and is not inherited or caused by anything one has or has not done.
There is a continuous irritation of the vein by the combination of pulsation by the artery with the compression, and this causes webs of scar tissue to form on the inside of the vein (trabeculae). They obstruct the flow of blood from the leg and pelvis back to the heart.
It is thought to occur in around 20-25% of people and is more common in women than men. Not everyone who has MTS will have problems related to it, but the percentage that does have problems is not known.