Heavy periods are most often caused by hormonal imbalances, structural growths in the uterus like fibroids or polyps, or conditions where uterine tissue grows where it shouldn’t. Clinically, a period is considered heavy when blood loss exceeds 80 milliliters per cycle, roughly equivalent to soaking through a pad or tampon every hour for several consecutive hours. But the causes behind that excess bleeding vary widely, and identifying yours is the key to getting it under control.
Hormonal Imbalances and Ovulation Problems
The most common reason for heavy periods is a disruption in the normal balance between estrogen and progesterone, the two hormones that control your menstrual cycle. During the first half of your cycle, estrogen thickens the uterine lining to prepare for a potential pregnancy. After ovulation, progesterone rises and stabilizes that lining. If pregnancy doesn’t happen, both hormones drop, and the lining sheds as your period.
When ovulation doesn’t occur, which is called anovulation, progesterone never kicks in. Without it, estrogen continues stimulating the uterine lining unopposed, causing it to grow thicker than normal. When that overgrown lining finally sheds, the result is heavier, often irregular bleeding. This pattern is especially common during two life stages: the first few years after periods begin (when cycles haven’t fully regulated) and the years leading up to menopause (when ovulation becomes less consistent).
Chronic estrogen dominance without adequate progesterone can also lead to a condition called endometrial hyperplasia, where the cells lining the uterus crowd together and become abnormal. This isn’t just a cause of heavy bleeding. Left untreated, certain types can progress to uterine cancer, which is why persistent heavy periods warrant investigation.
Fibroids and Polyps
Uterine fibroids are noncancerous growths in the muscular wall of the uterus. They’re extremely common, affecting up to 70-80% of women by age 50, though many never cause symptoms. When fibroids do cause heavy periods, it’s because they distort the uterine wall or the inner lining, increasing the surface area that bleeds during menstruation. Fibroids that grow into the uterine cavity (called submucosal fibroids) are the most likely to cause heavy flow.
Endometrial polyps are smaller, finger-like growths on the uterine lining itself. Like fibroids, they’re usually benign, but they disrupt the normal shedding process and create extra bleeding surface. Both fibroids and polyps can also interfere with the uterus’s ability to contract and compress blood vessels during your period, a natural mechanism your body uses to slow bleeding.
Adenomyosis
Adenomyosis is a condition where the tissue that normally lines the inside of the uterus grows into the muscular wall. During your period, that embedded tissue thickens, breaks down, and bleeds, just like the lining inside the uterus does. The difference is that it’s trapped within the muscle, causing the uterus to enlarge and triggering painful, heavy periods. Some people with adenomyosis have no symptoms at all, while others experience significantly prolonged or intense bleeding. It’s often diagnosed in women in their 30s and 40s, and it can coexist with endometriosis or fibroids, making the bleeding even worse.
Thyroid Problems
An underactive thyroid (hypothyroidism) is a frequently overlooked cause of heavy periods. The connection works through multiple pathways. Low thyroid hormone impairs platelet function, which reduces your blood’s ability to clot normally during menstruation. It also shifts the body toward a “hypocoagulable” state, meaning the overall balance of clotting factors tilts toward more bleeding. On top of that, hypothyroidism can disrupt ovulation, triggering the same estrogen-dominant pattern described above. Even subclinical hypothyroidism, where thyroid levels are only slightly off, can cause platelet dysfunction significant enough to increase menstrual flow.
Bleeding Disorders
This is one of the most underdiagnosed causes of heavy periods, particularly in younger women. A multicenter study of 200 adolescents with heavy menstrual bleeding found that 33% had an underlying bleeding disorder. The most common was low levels of von Willebrand factor (16%), followed by von Willebrand disease (11%) and platelet dysfunction (4.5%). Von Willebrand factor is a protein that helps blood clot. When levels are low, even normal menstrual bleeding becomes harder for the body to control.
Bleeding disorders are worth considering if your heavy periods started with your very first cycle, if you bruise easily, bleed heavily after dental work or minor cuts, or if close family members have similar issues. These conditions are genetic and lifelong, but very manageable once identified.
Other Contributing Factors
Several additional factors can cause or worsen heavy periods:
- Copper IUD: Non-hormonal intrauterine devices commonly increase menstrual flow, especially in the first 3 to 6 months after insertion.
- Blood-thinning medications: Anticoagulants reduce clotting and can make periods noticeably heavier.
- Polycystic ovary syndrome (PCOS): Irregular ovulation in PCOS leads to the same estrogen-dominant buildup of the uterine lining, often resulting in infrequent but very heavy periods.
- Early pregnancy complications: A miscarriage or ectopic pregnancy can present as an unusually heavy period, especially if you didn’t know you were pregnant.
How to Tell If Your Period Is Too Heavy
Most people don’t measure their blood loss in milliliters, so practical signs matter more than exact numbers. Your period is likely heavier than normal if you’re soaking through a pad or tampon in an hour or less for several hours in a row, passing blood clots larger than a quarter, needing to double up on protection (a pad and tampon together), or waking at night specifically to change your pad.
Tracking tools can help you quantify what you’re experiencing. One widely used method scores your used pads and tampons: a lightly stained item scores 1 point, a moderately soaked item scores 5, a fully saturated tampon scores 10, and a fully saturated pad scores 20. Small clots add 1 point each, larger clots add 5. A score above 100 for a full cycle correlates with blood loss over the 80-milliliter threshold. This kind of tracking gives you something concrete to bring to a medical appointment.
How Heavy Periods Affect Your Health
The biggest downstream risk of chronically heavy periods is iron deficiency anemia. Every period depletes your iron stores, and when bleeding is excessive, your body can’t replenish them fast enough. You’re considered iron deficient when your ferritin level drops below 30 ng/mL. Symptoms include fatigue that doesn’t improve with sleep, brain fog, shortness of breath during mild activity, hair thinning, and feeling cold all the time. Many women attribute these symptoms to stress or poor sleep without realizing their period is the root cause.
If you’ve been told your blood count is “normal” but still feel exhausted, ask specifically about your ferritin level. Standard blood tests check hemoglobin, which can remain in the normal range even after iron stores are significantly depleted. Ferritin catches the deficiency earlier.
What Doctors Look For
The diagnostic process for heavy periods focuses on three things: checking for anemia and low iron stores, identifying hormonal or ovulatory problems, and screening for bleeding disorders. Blood work typically includes a complete blood count, ferritin, and thyroid function tests. For adolescents, imaging like ultrasound isn’t routinely done upfront. It’s reserved for cases that don’t respond to initial treatment. In adults, ultrasound is more commonly used early on to check for fibroids, polyps, or signs of adenomyosis.
If a bleeding disorder is suspected, specialized clotting tests are ordered. This step is particularly important for teenagers and young women whose heavy periods began at menarche, since one in three adolescents with heavy periods turns out to have an underlying clotting issue. Endometrial biopsy may be recommended for women over 35 or those with risk factors for endometrial hyperplasia, to rule out abnormal cell changes in the uterine lining.