Heavy periods are most often caused by a hormonal imbalance that lets the uterine lining grow too thick before it sheds, but structural growths, underlying health conditions, and bleeding disorders can all play a role. Clinically, blood loss above 80 mL per cycle is considered heavy, though most people gauge it by practical signs like soaking through a pad or tampon every hour for several hours in a row. Understanding the cause matters because the right treatment depends entirely on what’s driving the bleeding.
Hormonal Imbalance and Ovulation Problems
The most common cause of heavy periods is a disruption in the hormonal cycle that controls how thick your uterine lining gets each month. Normally, estrogen builds the lining during the first half of your cycle, then progesterone stabilizes it after ovulation and triggers a controlled shed. If ovulation doesn’t happen, progesterone is never produced, and estrogen continues thickening the lining unopposed. When that overgrown lining finally breaks down, the bleeding is heavier and often irregular.
This pattern, called anovulation, is especially common during two life stages: the first few years after periods start and the years leading up to menopause. Polycystic ovary syndrome (PCOS) is another frequent cause of missed ovulation. Over time, prolonged estrogen exposure without progesterone can lead to endometrial hyperplasia, a condition where the cells lining the uterus crowd together and become abnormal. Hyperplasia isn’t cancer, but certain types can progress to it, which is one reason persistent heavy bleeding is worth investigating.
Fibroids and Polyps
Uterine fibroids are noncancerous muscular growths in the wall of the uterus. They’re extremely common, affecting up to 70–80% of women by age 50, though not all fibroids cause symptoms. The ones most likely to cause heavy bleeding are those that grow into or distort the inner cavity of the uterus, because they increase the surface area of the lining and interfere with the uterus’s ability to contract and stop bleeding after shedding.
Uterine polyps are different. These are softer growths that form when cells in the endometrium overgrow and attach to the inner wall of the uterus. Polyps can cause heavy flow, bleeding between periods, and irregular cycles. They range from a few millimeters to several centimeters in size. While most polyps are benign, a small percentage can contain precancerous or cancerous cells, particularly in women after menopause.
Adenomyosis
Adenomyosis occurs when the tissue that normally lines the uterus grows into the muscular wall of the uterus itself. Each month, that embedded tissue responds to hormones just like the regular lining, swelling and bleeding inside the muscle. This causes the uterus to enlarge, sometimes to two or three times its normal size, and makes periods significantly heavier and more painful.
Research comparing adenomyosis and endometriosis, a related condition where similar tissue grows outside the uterus, found a striking difference in bleeding patterns. Heavy menstrual bleeding affected 64% of women with adenomyosis compared to just 19% of those with endometriosis. So while endometriosis is more closely linked to pelvic pain and pain during sex, adenomyosis is the one that tends to dramatically increase flow. It’s most commonly diagnosed in women in their 30s and 40s, particularly those who have had children.
Bleeding Disorders
Some women bleed heavily because their blood doesn’t clot properly, and this cause is more common than most people realize. Von Willebrand disease, the most prevalent inherited bleeding disorder, affects an estimated 13% of women with heavy periods, based on a systematic review of nearly 1,000 women with menorrhagia. Many of these women go undiagnosed for years because heavy periods are often dismissed as “just how their body works.”
Clues that a bleeding disorder might be involved include heavy periods starting from your very first cycle, frequent nosebleeds, easy bruising, prolonged bleeding after dental procedures or cuts, and a family history of bleeding problems. If your heavy periods have been present since adolescence and don’t have an obvious structural or hormonal explanation, a blood clotting evaluation is a reasonable next step.
Thyroid Problems
An underactive thyroid (hypothyroidism) can cause heavy periods through a less obvious pathway. Low thyroid hormone levels shift the body’s clotting system toward a state where blood is slower to form clots and existing clots break down faster. Hypothyroidism also increases the risk of acquired von Willebrand syndrome, essentially mimicking the inherited bleeding disorder described above. The result is heavier, longer periods, often accompanied by fatigue, weight gain, and feeling cold, which are the more commonly recognized thyroid symptoms. Treating the thyroid problem frequently improves the bleeding.
Other Contributing Factors
Several other conditions and situations can cause or worsen heavy periods:
- Copper IUD: Non-hormonal intrauterine devices commonly increase menstrual flow by 20–50%, especially in the first several months after insertion.
- Medications: Blood thinners and regular use of anti-inflammatory drugs like aspirin can increase bleeding volume.
- Pregnancy complications: A very heavy, late period can sometimes be an early miscarriage or ectopic pregnancy, particularly if you’re also experiencing unusual cramping or clotting.
- Endometrial or cervical conditions: Rarely, heavy bleeding can signal precancerous changes or cancer of the uterus or cervix, which is why persistent changes in your bleeding pattern warrant evaluation.
How to Tell If Your Bleeding Is Too Heavy
Measuring actual blood loss isn’t practical at home, so the Cleveland Clinic offers useful benchmarks. You’re likely experiencing heavy bleeding if you soak through a pad or tampon every hour for several consecutive hours, need to double up on protection, pass blood clots larger than a quarter, or find that bleeding regularly lasts more than seven days. Soaking through two or more pads or tampons per hour for two to three hours straight is a sign to seek immediate medical attention.
Waking up at night to change protection, avoiding activities because of your flow, or feeling exhausted and lightheaded during your period are also signals worth paying attention to. These symptoms can point to iron deficiency anemia, which develops gradually when monthly blood loss outpaces your body’s ability to replace red blood cells.
The Iron Deficiency Connection
Chronic heavy periods are the leading cause of iron deficiency in premenopausal women, and the relationship becomes a cycle: heavy bleeding depletes iron, and low iron can itself worsen bleeding by impairing the uterine muscle’s ability to contract. Iron deficiency is diagnosed when ferritin, the protein that stores iron, drops below 30 micrograms per liter. Anemia is identified when hemoglobin falls below 120 g/L in premenopausal women.
The tricky part is that you can be significantly iron-depleted before your hemoglobin drops enough to flag as anemia on a standard blood test. Symptoms like fatigue, brain fog, hair loss, restless legs, and shortness of breath during exercise can appear well before you’re technically anemic. If you have heavy periods and any of these symptoms, asking specifically for a ferritin test (not just a complete blood count) gives a clearer picture.
How Heavy Bleeding Is Evaluated
If you bring up heavy periods with your doctor, the evaluation typically starts with your history: how long your periods last, how many products you go through, whether your cycles are regular, and whether you have symptoms of anemia or a bleeding disorder. Blood work usually includes a complete blood count, iron studies, thyroid function, and sometimes clotting tests.
The next step depends on what the history suggests. If a structural problem like fibroids or polyps is suspected, a transvaginal ultrasound is the standard first imaging tool. If the ultrasound shows an abnormal cavity or if the picture is unclear, a more detailed look using saline-infused sonography or a hysteroscopy (a thin camera inserted through the cervix) can identify specific growths. For women with risk factors for endometrial hyperplasia or cancer, such as prolonged irregular bleeding, obesity, or age over 45, an endometrial biopsy may be recommended to check the lining cells directly.
Finding the cause sometimes takes more than one visit, especially when multiple factors overlap. A woman with fibroids and a thyroid problem, for instance, may need both addressed before her periods normalize. The good news is that nearly every cause of heavy bleeding has effective treatment options, ranging from hormonal therapies and targeted procedures to simply correcting an underlying condition like hypothyroidism.