Uterine fibroids cause heavy bleeding through several reinforcing mechanisms: they expand the surface area of the uterine lining, interfere with the uterus’s ability to contract and slow blood flow, and drive abnormal blood vessel growth in surrounding tissue. The result is periods that last longer than seven days and produce roughly twice the normal blood loss. Not all fibroids cause bleeding equally. Location, size, and how deeply a fibroid presses into the uterine cavity all determine whether it disrupts your cycle.
How Fibroids Expand the Bleeding Surface
During a normal period, the inner lining of the uterus sheds and the exposed blood vessels bleed briefly before the muscle wall contracts to compress them shut. A fibroid, particularly one that bulges into the uterine cavity, stretches that lining over a larger area. More surface area means more blood vessels are exposed when the lining sheds, and more blood is lost before the process completes.
This stretching also physically distorts uterine cells, altering how they function at a molecular level. The lining tissue near a fibroid doesn’t behave the same as healthy endometrium further away. It produces different signaling molecules and responds differently to hormones, which compounds the bleeding problem beyond simple surface area.
Disrupted Muscle Contractions
The muscular wall of the uterus plays an active role in controlling menstrual blood loss. During your period, rhythmic contractions squeeze blood vessels shut and help expel the shed lining efficiently. Fibroids, especially large ones embedded in the muscle wall (intramural fibroids) or those pressing inward from below the lining (submucosal fibroids), physically obstruct this process.
A uterus with fibroids contracts abnormally. The muscle can’t generate uniform pressure when a dense, fibrous mass is sitting in its wall. The contractions become uncoordinated and less effective at compressing open blood vessels. This leads to both heavier flow and periods that drag on for days longer than they should, because the uterus simply can’t shut down the bleeding as quickly.
Abnormal Blood Vessel Growth
Fibroids don’t just passively sit in the uterus. They actively reshape the blood supply around them. Because oxygen levels inside fibroids are significantly lower than in the surrounding muscle, the fibroid tissue responds by ramping up production of signals that trigger new blood vessel formation. Research published in Fertility and Sterility found that fibroid cells in low-oxygen conditions increased their output of a key vessel-growth signal (VEGF-A) by over four-fold compared to normal oxygen conditions.
This surge in new vessel growth creates a denser, more fragile network of blood vessels in and around the fibroid. These vessels are structurally weaker than the well-organized vessels in healthy uterine tissue. When the lining sheds during your period, these abnormal vessels bleed more readily and are harder for the body to seal off.
Venous Congestion and Trapped Blood
Fibroids also cause problems on the drainage side. As a fibroid grows within the uterine wall, it increases pressure on the veins that normally carry blood away from the lining. This impaired drainage leads to a condition called endometrial venous ectasia, where veins near the surface of the lining become dilated and congested with slow-moving blood.
Slow blood flow through dilated veins sets up a cascade of problems. The sluggish flow damages the vessel walls, and the stagnant blood becomes prone to clotting within the vessel itself. When these damaged, engorged veins eventually rupture during menstruation, they release a larger volume of blood than healthy veins would. This is one reason women with fibroids often pass large clots during their period. The combination of slow flow, vessel wall injury, and a tendency toward clotting within the veins (a pattern pathologists call Virchow’s triad) makes these dilated vessels a persistent source of heavy bleeding.
Which Fibroids Cause the Most Bleeding
Location matters more than size. Submucosal fibroids, which grow just beneath the uterine lining and push into the cavity, are the most likely to cause heavy bleeding even when they’re relatively small. A submucosal fibroid the size of a marble can cause more bleeding than an intramural fibroid the size of a grapefruit, because it directly distorts the lining and its blood supply.
Intramural fibroids (embedded in the muscle wall) cause significant bleeding when they’re large enough to distort the cavity shape or when they compress the veins draining the lining. Subserosal fibroids, which grow on the outer surface of the uterus, rarely cause heavy periods at all because they don’t interact with the lining or its blood vessels. If you’ve been told you have fibroids but your periods are normal, the fibroids are likely in a location that doesn’t affect the inner cavity.
How Heavy Is Too Heavy
A typical period produces about 2 to 3 tablespoons of blood over 4 to 5 days. Heavy menstrual bleeding, by the CDC’s definition, means bleeding for more than 7 days or losing roughly twice that normal amount. Women with symptomatic fibroids often far exceed this threshold.
Tracking your bleeding can help you communicate with a healthcare provider. One validated approach is the menstrual pictogram: a visual chart with images showing different levels of saturation on pads or tampons. Each time you change a product, you match it to the image that looks closest. Digital versions of this tool now exist as apps, making it easy to log your data over several cycles and share it directly. Patterns like soaking through a pad or tampon every hour for several consecutive hours, passing clots larger than a quarter, or needing to double up on products are all signs your bleeding has crossed into heavy territory.
The Anemia Connection
Chronic heavy bleeding drains your body’s iron stores. Red blood cells need iron to carry oxygen, and when you’re losing more blood each month than your body can replace, iron-deficiency anemia develops. Among women with symptomatic fibroids, anemia is extremely common. A study at a regional referral hospital found that nearly two-thirds of women with fibroids were anemic, with about 29% classified as severely anemic.
Anemia from fibroid-related bleeding builds gradually, so you may not notice it until it’s significant. Fatigue that doesn’t improve with rest, shortness of breath during activities that used to feel easy, dizziness when standing up, and unusual cravings for ice or non-food items are classic signs. Because the blood loss happens slowly over many cycles, your body partially adapts, which means you can reach surprisingly low iron levels before you feel dramatically unwell. If your periods have been getting heavier over months or years, a simple blood test for iron levels and hemoglobin can reveal whether your body is keeping up with the losses.