The underlying anatomy behind May-Thurner syndrome is surprisingly common, present in roughly 20% to 25% of the general population. But only 1% to 5% of those people ever develop symptoms. That distinction between having the anatomy and having the syndrome is key to understanding how common this condition really is.
What May-Thurner Syndrome Actually Is
In your pelvis, the right iliac artery (which sends blood to your right leg) crosses over the left iliac vein (which returns blood from your left leg back to your heart). In most people, this crossover causes no issues. In May-Thurner syndrome, the artery presses down on the vein hard enough to narrow it, slowing blood flow and raising the risk of a blood clot forming in the left leg.
This is why left-sided deep vein thrombosis (DVT) is the hallmark of the condition. May-Thurner syndrome accounts for an estimated 2% to 5% of all DVT cases, but autopsy studies suggest the true prevalence of the compression is much higher than clinical diagnoses reflect. Many people live with significant compression and never know it.
How Common Is the Compression Itself?
A large cross-sectional study from southern China measured the left iliac vein in 896 people with no symptoms. Some degree of compression was nearly universal, with the average compression measuring 44%. When researchers categorized severity, 62% of people had mild compression, 28% had moderate compression, and about 10% had severe compression. None of these people had symptoms.
This is an important point: having a compressed vein doesn’t mean you have May-Thurner syndrome. The compression only becomes a clinical problem when it causes symptoms like leg swelling, pain, skin changes, or a DVT. The gap between anatomical compression (very common) and symptomatic disease (uncommon) is enormous.
Who Gets Diagnosed Most Often
May-Thurner syndrome is diagnosed about twice as often in women as in men. A systematic review of over 1,400 patients found a female-to-male ratio of roughly 2 to 1, with women making up 67% of cases. Women also tend to be diagnosed younger, at an average age of about 39, compared to 46 for men.
The condition typically shows up between the second and fourth decades of life, so most patients are in their 20s through 40s. Pregnancy is a major trigger. During pregnancy, the risk of blood clots rises four- to fivefold compared to the non-pregnant state, and in the third trimester, the enlarged uterus can press on the left iliac vein from a different angle, adding compression on top of the existing anatomical crossover. The postpartum period carries elevated risk as well. Hormonal changes during pregnancy increase the blood’s tendency to clot, which compounds the mechanical problem of a narrowed vein.
Symptoms to Recognize
Most people with May-Thurner anatomy have no symptoms at all. When the compression does cause problems, the most common presentation is a DVT in the left leg, which feels like sudden swelling, warmth, and pain, usually in the thigh or calf. The swelling is typically one-sided, affecting only the left leg.
Some people develop chronic symptoms without a full clot. These can include persistent left leg swelling that worsens throughout the day, a feeling of heaviness or aching, visible varicose veins, or skin discoloration around the ankle. Because these symptoms overlap with many other vein conditions, May-Thurner syndrome is frequently underdiagnosed or caught only after someone has already had a clot.
How It’s Treated
When May-Thurner syndrome causes a DVT, the immediate priority is clearing the clot and restoring blood flow. The most common long-term fix is placing a small metal stent inside the compressed vein to hold it open. This is done through a catheter, not open surgery, and typically involves a short hospital stay.
Stent outcomes are generally good. Studies report that the stent stays open in about 78% to 90% of patients at one year, and around 78% at two years. These numbers vary depending on whether the patient had an acute clot, a chronic blockage, or compression without clotting, but overall the procedure has a strong track record for relieving symptoms and preventing recurrence.
For people with the anatomical compression but no symptoms and no history of clots, treatment is typically not recommended. One study of 68 women with a history of left-sided DVT found that May-Thurner anatomy alone did not increase the rate of long-term complications like chronic pain, swelling, or skin changes after a clot. About 37% of the group developed these post-clot symptoms, but the compression itself wasn’t the driving factor. This finding has led some specialists to question whether aggressive treatment of the compression is necessary when it isn’t actively causing problems.
Why It’s Likely Underdiagnosed
The gap between how common the anatomy is (20% to 25% of people) and how rarely it’s formally diagnosed points to significant underrecognition. Standard imaging for a DVT, like a basic ultrasound, can identify a clot but often can’t see the pelvic veins well enough to detect the compression behind it. More advanced imaging, such as CT venography or intravascular ultrasound, is needed to confirm the diagnosis, and these aren’t always ordered unless a doctor specifically suspects the condition.
If you’ve had a left-sided DVT, especially if you’re a younger woman without obvious risk factors like recent surgery or prolonged immobility, May-Thurner syndrome is worth asking about. Recurrent left-sided clots are a particularly strong signal. The condition is treatable and the stenting procedure is well-established, but the diagnosis has to be considered in the first place for it to be found.