Large uterine fibroids can cause blood clots, primarily by physically pressing on the major veins in the pelvis and slowing blood flow enough to trigger clot formation. The relationship between fibroids and venous thromboembolism, which includes deep vein thrombosis (DVT) and pulmonary embolism (PE), is increasingly recognized in medical literature but still underappreciated in routine clinical care. The risk depends heavily on fibroid size, location, and the individual anatomy of the person affected, and the clinical picture gets complicated when the same fibroids also cause heavy bleeding.
How Fibroids Lead to Blood Clots
The main mechanism is straightforward: a large fibroid sitting in the pelvis can physically compress the veins that carry blood back up from the legs. The iliac veins and the inferior vena cava (the body’s largest vein, running up the center of the abdomen) are the most commonly affected. When a fibroid presses against these vessels, blood flow slows or partially stops, creating the stagnant conditions that allow clots to form. This process is called venous stasis, and it follows the same basic logic as why sitting on a long flight raises your clot risk, except the compression is internal and constant.
An expert review published in 2024 confirmed that the relationship between fibroids and venous thromboembolism is mainly attributed to this physical compression of pelvic veins, particularly the iliac veins.
Case reports illustrate how dramatic this compression can get. In one documented case, large fibroids completely compressed the distal inferior vena cava and both ureters, causing the vein above the obstruction to dilate to 27 millimeters. The patient underwent a hysterectomy, and an eight-pound uterus was removed. Intraoperative imaging afterward showed the vein was patent with adequate blood flow, and her lower-extremity symptoms resolved.
Size Is the Biggest Risk Factor
Not all fibroids carry clot risk. Small fibroids, which are extremely common and often go unnoticed, are unlikely to compress anything large enough to matter. The risk rises sharply with size. A Japanese study that examined DVT rates in patients with fibroids found a clear threshold: the rate of DVT was about 11.5 percent when the uterus weighed 1,000 grams or more, compared with 3 percent when it weighed less than that. The researchers described that 1,000-gram cutoff as roughly “adult head size on pelvic examination.”
That comparison is helpful for perspective. Most fibroids never get anywhere near that large. But for the subset of patients whose fibroids do grow to that extent, the clot risk is real and roughly four times higher than in patients with smaller fibroids. Location matters too: a fibroid sitting on the back wall of the uterus is more likely to press against the vena cava, while one on the side may compress the iliac veins differently depending on its exact position.
When Anatomy Makes Things Worse
Some people have a pre-existing anatomical quirk called May-Thurner syndrome, in which the left iliac vein is already partially compressed by the right iliac artery crossing over it. This condition is more common than most people realize, and it often goes undiagnosed until something triggers a clot. A large fibroid adding compression on top of an already-narrowed vein can tip the balance.
One case report described a patient whose DVT was confined entirely to the left leg, even though her fibroid had shifted to the right side. The explanation was that she had underlying May-Thurner anatomy on the left, so the combination of the pre-existing compression and the fibroid’s pressure created the conditions for a clot only on that side. If the clot had been caused purely by the fibroid pressing on the inferior vena cava, the researchers noted, you would expect both sides to be affected.
Another report described a pregnant patient with a large fibroid that worsened lower-extremity venous stasis in a pattern resembling May-Thurner syndrome. In pregnancy, the uterus is already growing and putting pressure on pelvic veins, so adding a large fibroid to the mix compounds the risk significantly.
The Bleeding-Versus-Clotting Paradox
Here is where fibroids create a genuinely difficult clinical problem. The most common symptom of fibroids is heavy menstrual bleeding. Many patients are anemic, sometimes severely, because of chronic blood loss. The instinct in treating heavy bleeding is to use medications that help the blood clot more effectively, like tranexamic acid or hormonal therapies. But if the same patient also has large fibroids compressing her pelvic veins, giving procoagulant medications could raise the risk of a dangerous venous clot.
A case report examining this exact dilemma warned that medical management of heavy bleeding with procoagulants should be avoided in women whose large fibroids put them at higher risk of developing venous thromboembolism. The challenge is that many clinicians treating the bleeding may not be thinking about the clot risk, and vice versa.
On the flip side, if a patient develops a DVT and is started on blood thinners, those anticoagulants can worsen the already-heavy menstrual bleeding caused by the fibroids. One review noted that anticoagulant use in patients with heavy menstrual bleeding creates a genuine debate about bleeding risks and the need for careful coordination. The authors recommended considering mechanical prevention methods like compression stockings or using anticoagulants that can be more easily reversed before any necessary surgery.
This dual problem, bleeding that demands clotting support and clot risk that demands blood thinners, requires a team approach. Current guidance emphasizes individualized surgical planning, prompt anticoagulation when clots are present, and multidisciplinary coordination between gynecologists and vascular specialists.
Anemia Itself Can Contribute to Clot Risk
The connection between fibroids and clots runs through another unexpected pathway: iron deficiency anemia. When fibroids cause chronic heavy bleeding, the resulting anemia can itself change the blood in ways that promote clotting. Researchers have reported cases of acute ischemic stroke in middle-aged women whose strokes were linked to iron deficiency anemia from fibroid-related bleeding. The proposed mechanisms include an increase in platelet count (the body produces more platelets when iron is low), a shift toward a hypercoagulable state, and tissue-level oxygen deprivation from the anemia.
This is not the most common scenario, but it highlights how fibroids can contribute to clot-related events even when the fibroids themselves are not large enough to physically compress a vein. Chronic, untreated heavy bleeding creates its own cascade of risks that go beyond just feeling tired or short of breath.
What Happens When Fibroids Degenerate
Fibroids sometimes outgrow their blood supply and begin to break down internally, a process called degeneration. The most dramatic form, red degeneration, involves hemorrhagic infarction and tissue death within the fibroid itself. This process is often triggered by blood clots forming in the small veins at the fibroid’s periphery, cutting off its circulation.
In rare cases, a degenerating fibroid can release enough tissue breakdown products into the bloodstream to trigger a body-wide clotting disorder called disseminated intravascular coagulation (DIC). One case report described a patient whose large degenerating fibroid caused DIC with excessive clot breakdown. After hysterectomy, the removed tissue showed extensive clotting within the tumor itself, and the patient’s clotting levels returned to normal without further complications. DIC is a medical emergency regardless of its cause, so this is an important link to recognize even though it is uncommon.
Surgical and Procedural Clot Risks
Any pelvic surgery carries some risk of blood clots, and fibroid-related procedures are no exception. A population-based study from South Korea found that venous thromboembolism after myomectomy (surgical removal of fibroids while preserving the uterus) occurred at a rate of about 5.7 per 10,000 patients, with DVT at 4.4 per 10,000 and pulmonary embolism at 2.5 per 10,000. Those numbers are low in absolute terms, and they include standard post-surgical prevention measures like blood thinners and early mobilization.
Uterine artery embolization (UAE), a minimally invasive procedure that cuts off blood flow to fibroids to shrink them, also carries clot risk. A systematic review and meta-analysis found that DVT or PE occurred in about 0.2 percent of UAE procedures. While that rate is low, the consequences can be severe. At least one fatal pulmonary embolism has been reported in the United Kingdom following UAE, in a 44-year-old woman who went into cardiac arrest 19 hours after the procedure. A broader review of UAE complications lists thromboembolic events among the acute post-procedural risks alongside infection and pain.
The takeaway is not that these procedures are dangerous, as both are considered safe overall, but that patients with large fibroids may already be in a higher-risk state for clots going into surgery, and that appropriate prevention measures matter.
A Rare Blood Abnormality Worth Knowing About
In uncommon cases, fibroids can cause the opposite of anemia: an abnormal increase in red blood cells. Some fibroids secrete erythropoietin (EPO), the same hormone the kidneys produce to stimulate red blood cell production. When a fibroid produces EPO on its own, the body makes too many red blood cells, thickening the blood and potentially raising clot risk through a completely different mechanism than venous compression.
This condition is rare enough that it shows up primarily in case reports rather than large studies. But it is worth flagging because a patient found to have an unexplained rise in red blood cell counts alongside fibroids should have this possibility considered, especially if the more common causes of elevated red blood cells have been ruled out.
Medications That Shrink Fibroids and Reduce Bleeding
For patients caught in the bleeding-versus-clotting bind, shrinking the fibroids medically can address both problems at once. A class of drugs called GnRH antagonists (such as relugolix combined with low-dose hormones) has shown effectiveness in controlling fibroid-related bleeding and modestly reducing fibroid size. A meta-analysis found that GnRH antagonists were about five times more likely to control uterine bleeding compared to placebo, with a meaningful reduction in fibroid volume.
In the LIBERTY trials, one such combination achieved continued fibroid volume reduction over a year, with the effect growing from about 13.5 percent shrinkage at six months to roughly 18 percent at one year. While those reductions are modest, the bleeding control tends to be more dramatic and immediate, which can break the cycle of anemia and its downstream clotting effects. These medications also buy time for patients who need surgery but are not yet stable enough for it, or who want to avoid surgery altogether.
Tranexamic acid, the most commonly used non-hormonal medication for heavy periods, has a better safety profile for clotting than its reputation sometimes suggests. A systematic review of its use in heavy menstrual bleeding found no thromboembolic events across all the studies analyzed. That said, the case reports urging caution about procoagulants in patients with very large fibroids still apply: a drug’s general safety profile may not extend to someone whose pelvic veins are already partially compressed. Context matters more than blanket rules.
Who Should Be Thinking About This
Most people with fibroids will never develop a blood clot because of them. The risk concentrates in a specific population: those with very large fibroids, particularly fibroids that are growing rapidly or have reached the point where they are causing pelvic pressure symptoms like leg swelling, difficulty urinating, or a feeling of heaviness in the pelvis. If you have fibroids and notice swelling, pain, or warmth in one leg, those are classic DVT warning signs that deserve urgent evaluation regardless of whether you connect them to your fibroids.
The broader message is that fibroids are not just a gynecological issue. When they get large enough, they become a vascular issue as well. That crossover means that ideal care involves more than one specialty, and patients benefit from making sure all their providers know the full picture. A gynecologist managing fibroid bleeding should be aware of any history of blood clots or leg swelling, and a vascular specialist treating a DVT in a woman of reproductive age should consider whether large fibroids might be contributing to venous compression.
Fibroids and Clot Risk During Pregnancy
Pregnancy already raises clot risk because of hormonal changes, increased blood volume, and the growing uterus pressing on pelvic veins. Fibroids can compound every one of those factors. A fibroid that was manageable before pregnancy may grow during gestation due to higher estrogen levels, potentially crossing the size threshold where venous compression becomes clinically meaningful. The combination of pregnancy-related hypercoagulability and fibroid-related venous stasis can create a particularly high-risk scenario.
Treatment options are limited during pregnancy since surgery carries its own risks to the pregnancy, and not all anticoagulant medications are safe for the developing fetus. Low-molecular-weight heparin is generally the anticoagulant of choice when clot prevention is needed in pregnant patients, but the decision to use it requires weighing the bleeding risks from both the fibroids and the pregnancy itself. Compression stockings and early mobilization become even more important preventive measures in this population.
For patients with known large fibroids who are planning pregnancy, discussing fibroid management before conception is worthwhile. Shrinking fibroids or removing them beforehand can reduce both the bleeding and clot-related complications that might arise once pregnancy limits the available treatment options.