May-Thurner syndrome is caused by a quirk of anatomy in your pelvis: the right iliac artery, which carries blood down to your right leg, crosses over the left iliac vein, which carries blood back up from your left leg. In some people, that artery presses hard enough on the vein to partially block blood flow, like stepping on a garden hose. Around 20 to 25 percent of the general population has this overlapping anatomy, but only 1 to 5 percent of them ever develop symptoms.
The Anatomy Behind the Compression
Deep in the pelvis, the right common iliac artery and the left common iliac vein sit in close quarters. The artery passes directly over the vein, and behind the vein sits the fifth lumbar vertebra, a solid bone of the lower spine. The vein is essentially sandwiched between a pulsing artery in front and an unyielding bone behind. In most people, this causes no trouble. But when the artery compresses the vein firmly enough against that vertebra, blood flow from the left leg slows down, and problems can follow.
This is why May-Thurner syndrome almost always affects the left leg. The anatomical crossover happens on the left side in the vast majority of people. Right-sided cases are rare because the corresponding vessels on the right side of the pelvis don’t typically cross in the same way.
How the Damage Builds Over Time
The artery doesn’t just sit on the vein. It pulses with every heartbeat, delivering a rhythmic mechanical stress to the vein wall underneath. Over months and years, this constant pulsation irritates the inner lining of the vein, triggering a biological repair response. Cells in the vein wall begin to proliferate, and the body deposits collagen and elastin, the same structural proteins found in scar tissue.
This process can eventually produce small ridges of tissue inside the vein called spurs. These spurs narrow the channel further, compounding the compression from the artery above. In some people, the combination of external compression and internal spur formation narrows the vein enough to significantly obstruct blood flow. That obstruction raises the risk of a deep vein thrombosis (DVT), a blood clot in the leg, which is often how May-Thurner syndrome first comes to medical attention.
Who Is Most Affected
Women develop symptomatic May-Thurner syndrome roughly twice as often as men. In a large systematic review of over 1,400 patients, the female-to-male ratio was about 2 to 1. Women also tend to present younger, at an average age of about 39, compared to 46 for men. The overall average age at diagnosis is around 43, though cases have been documented in teenagers and even children.
The reasons for the gender difference aren’t entirely settled, but hormonal factors play a role. Estrogen-containing oral contraceptives increase clotting risk on their own, and when combined with a vein that’s already partially compressed, they can tip the balance toward clot formation. Pregnancy is another significant trigger. The growing uterus puts additional pressure on pelvic veins, and pregnancy itself shifts the blood toward a more clot-prone state. These overlapping risks help explain why many women are diagnosed during or shortly after pregnancy.
Risk Factors That Trigger Symptoms
Having the anatomical compression is necessary but not sufficient. Most people with May-Thurner anatomy never know it because their blood flows well enough despite the narrowing. Symptoms tend to surface when additional risk factors stack on top of the existing compression.
- Pregnancy and postpartum recovery: increased pelvic pressure and a natural shift toward easier blood clotting combine to raise DVT risk in an already compromised vein.
- Hormonal contraceptives: estrogen-based birth control pills independently raise clotting risk, which becomes more dangerous in a narrowed vein.
- Prolonged immobility: long bed rest, surgery recovery, or extended travel slows venous blood flow, giving clots more opportunity to form in a vein that already has reduced flow.
- Dehydration: thicker, more concentrated blood is more prone to clotting, and the effect is amplified in a vein with sluggish circulation.
In many cases, it takes a combination of the underlying anatomy and one or more of these triggers to produce a clinical event. A person might carry May-Thurner anatomy for decades with no issues, then develop a DVT after a long hospitalization or during their first pregnancy.
What Symptoms Look Like
Because the compression affects the left leg’s venous drainage, symptoms show up almost exclusively in the left leg. The most common first sign is sudden swelling, pain, and warmth in the left leg from a DVT. Some people experience chronic symptoms before a clot ever forms: persistent leg swelling that worsens throughout the day, a feeling of heaviness or aching in the left leg, or visible varicose veins that are noticeably worse on one side. Skin changes like discoloration or ulcers near the ankle can develop if venous pressure remains elevated over a long period.
The one-sided nature of the symptoms is a key clue. When swelling or a DVT occurs only in the left leg, especially in a younger woman without obvious risk factors, May-Thurner syndrome is high on the list of possible explanations.
Why It Often Goes Undiagnosed
Standard imaging for a DVT, like an ultrasound of the leg veins, can confirm the clot but won’t necessarily reveal the underlying compression in the pelvis. The iliac veins sit deep in the body where ultrasound doesn’t always reach well. More specialized imaging, such as CT venography, MR venography, or an intravascular ultrasound (a tiny ultrasound probe threaded inside the vein), is typically needed to see the compression and measure how much the vein has narrowed.
Because many clinicians treat a left-leg DVT without looking deeper, May-Thurner syndrome can be missed entirely. The clot gets treated, but the structural cause remains, which can lead to recurrent clots or chronic swelling. If you’ve had a DVT in your left leg, particularly without a clear trigger like recent surgery, it’s worth asking whether pelvic vein compression could be a contributing factor.