Heavy menstrual bleeding has a wide range of causes, from growths in the uterus to hormonal imbalances to inherited bleeding disorders. Clinically, it’s defined as losing more than 80 milliliters of blood per cycle or bleeding that lasts longer than seven days. In practical terms, if you’re soaking through a pad or tampon every hour or two, passing large clots, or finding that your period disrupts your daily life, your bleeding qualifies as heavy.
The causes fall into two broad categories: structural problems (physical changes in the uterus) and non-structural problems (hormonal, systemic, or medication-related). Most are treatable once identified.
Uterine Growths: Fibroids and Polyps
Fibroids (also called leiomyomas) are the most common structural cause. These are noncancerous muscle growths in the wall of the uterus. Not all fibroids cause heavy bleeding. The ones that do are typically the type that push into the uterine cavity, distorting its lining and increasing the surface area that sheds each month. Fibroids can also interfere with the uterus’s ability to contract and squeeze blood vessels shut during a period, which is one of the body’s natural ways of slowing menstrual flow.
Uterine polyps are smaller, softer growths that develop on the inner lining of the uterus. They’re made of endometrial tissue rather than muscle. Polyps tend to cause bleeding between periods as well as heavier flow during periods. Both fibroids and polyps become more common as you get older and are readily visible on ultrasound.
Adenomyosis
Adenomyosis happens when the tissue that normally lines the uterus starts growing into the muscular wall. This causes the uterus to enlarge, sometimes significantly. The heavy bleeding it produces has a straightforward explanation: the endometrial lining ends up with a larger surface area, a greater total volume of glandular tissue, and increased blood supply. The deeper the tissue invades into the muscle wall, the heavier the bleeding tends to be. Adenomyosis also commonly causes severe cramping, because the misplaced tissue swells and bleeds within the muscle during each cycle. It’s most often diagnosed in women in their 30s and 40s, particularly those who have had children.
Hormonal and Ovulation Problems
Your menstrual cycle depends on a precise back-and-forth between estrogen and progesterone. Estrogen builds the uterine lining in the first half of the cycle; progesterone stabilizes it in the second half after ovulation. When you don’t ovulate (a cycle doctors call “anovulatory”), your body keeps producing estrogen without the balancing effect of progesterone. The lining keeps thickening, eventually becoming unstable, and when it finally sheds, the result is unpredictable, often heavy bleeding.
Anovulatory cycles are especially common at two points in life: the first few years after periods begin and the years leading up to menopause (perimenopause). Polycystic ovary syndrome (PCOS) is another frequent cause of irregular ovulation. In all of these situations, the underlying issue is the same: estrogen goes unopposed, the lining overgrows, and bleeding becomes heavy or prolonged.
Thyroid Disease
An underactive thyroid can cause heavy periods through a chain reaction of hormonal disruption. When the thyroid produces too little hormone, the pituitary gland ramps up its signaling to compensate. That increased pituitary activity spills over into other hormonal pathways. It raises prolactin levels and suppresses luteinizing hormone, which is essential for ovulation. The net effect is that estrogen stays elevated without progesterone to counter it, the uterine lining overgrows, and heavy breakthrough bleeding follows. If your heavy periods come alongside fatigue, weight gain, or feeling cold all the time, thyroid function is worth checking.
Bleeding Disorders
Some people bleed heavily because their blood doesn’t clot properly. The most common inherited bleeding disorder linked to heavy periods is von Willebrand disease, a condition where the blood is missing or low on a specific clotting protein. Among women with chronic heavy menstrual bleeding, somewhere between 5% and 24% turn out to have von Willebrand disease, depending on the population studied. It appears to be significantly more prevalent among white women (around 16%) than Black women (around 1%).
Von Willebrand disease often goes undiagnosed for years because heavy periods get treated as normal or attributed to other causes. Clues that a bleeding disorder might be involved include heavy periods that started from your very first cycle, a history of easy bruising, prolonged bleeding after dental work or surgery, or a family history of bleeding problems. Platelet disorders and rarer clotting factor deficiencies can also play a role.
Medications That Increase Flow
Several types of medication can make periods heavier as a side effect. Blood thinners (anticoagulants like warfarin) and antiplatelet drugs (like aspirin and clopidogrel) work by reducing the blood’s ability to clot, and that same mechanism makes menstrual bleeding harder for the body to control. If you’ve been started on any of these medications and notice a significant change in your periods, that connection is likely real.
Copper IUDs are another well-known cause. Unlike hormonal IUDs, which typically lighten periods, copper IUDs tend to increase menstrual flow, especially in the first several months after insertion. Some hormonal medications, including certain types of hormone replacement therapy, can also trigger irregular or heavy bleeding.
Endometrial Cancer and Precancer
Abnormal thickening of the uterine lining (endometrial hyperplasia) can progress to endometrial cancer in some cases, and heavy or irregular bleeding is often the earliest symptom. This is more of a concern after age 45, in people with obesity, those with a long history of irregular cycles, or anyone experiencing bleeding after menopause. Endometrial cancer is not a common cause of heavy periods overall, but it’s the reason doctors take new or changing bleeding patterns seriously, especially later in life.
How to Tell When Bleeding Is Too Heavy
Tracking your bleeding is helpful before any medical visit. Pay attention to how often you change pads or tampons, whether you pass clots (and how large), and whether bleeding extends past seven days. Signs that warrant prompt medical attention include soaking through at least one pad or tampon per hour for more than two consecutive hours, bleeding between periods, and any vaginal bleeding after menopause.
Over time, heavy periods can lead to iron-deficiency anemia. If you feel unusually tired, short of breath during normal activity, dizzy, or lightheaded, those symptoms may reflect blood loss that has depleted your iron stores. Anemia from heavy periods develops gradually, so many people adapt to feeling run down without realizing their energy levels have dropped far below normal. A simple blood test can confirm whether your iron levels are low, and treating the anemia alongside the underlying cause of heavy bleeding makes a meaningful difference in how you feel day to day.