Heavy periods have several possible causes, ranging from hormonal shifts to structural changes in the uterus to underlying medical conditions. Clinically, a period is considered heavy when you lose more than 80 milliliters (about 2.7 ounces) of blood per cycle, or when bleeding lasts longer than seven days. In practical terms, that looks like soaking through a pad or tampon every hour or two, or regularly passing blood clots larger than a quarter.
Hormonal Imbalance
The most common cause of heavy periods is an imbalance between estrogen and progesterone, the two hormones that control your menstrual cycle. During the first half of your cycle, estrogen drives the uterine lining to thicken and grow. In the second half, progesterone steps in to stop that growth and prepare the lining for either a pregnancy or a controlled shed. When progesterone is too low relative to estrogen, the lining keeps building unchecked, and there’s simply more tissue to shed when your period arrives.
This imbalance is especially common during puberty, in the years leading up to menopause, and in people with polycystic ovary syndrome (PCOS). Any cycle where you don’t ovulate, called an anovulatory cycle, skips the progesterone surge entirely. Without that counterbalance, estrogen continues stimulating the uterine lining for weeks. The result is a late, heavy, and often unpredictable period.
Uterine Fibroids
Fibroids are noncancerous growths in or on the uterine wall, and they’re one of the most well-known structural causes of heavy bleeding. They affect the majority of women by age 50, though not everyone with fibroids has symptoms. The ones most likely to cause heavy periods are submucosal fibroids, which grow into the uterine cavity, and intramural fibroids, which sit within the muscular wall.
Fibroids increase bleeding through several overlapping mechanisms. They enlarge the inner surface area of the uterus, meaning more lining tissue develops and more blood vessels are exposed during your period. They also interfere with the uterus’s ability to contract and clamp down on bleeding vessels the way it normally would. Perhaps most importantly, fibroids trigger chaotic new blood vessel formation in surrounding tissue, similar to what’s seen around tumors. These vessels are structurally fragile and prone to breaking and leaking. Older research also described “venous lakes,” dilated blood vessels compressed by the growing fibroid, where normal clotting mechanisms can’t seal off the larger-diameter vessels effectively.
Fibroids also appear to alter the local chemical environment. They secrete signaling molecules that reduce the lining’s ability to form proper clots, essentially thinning the blood’s ability to stop itself at the source.
Adenomyosis
Adenomyosis occurs when tissue that normally lines the uterus grows into the muscular wall itself. It’s a common but frequently underdiagnosed condition, particularly in women in their 30s and 40s. The hallmark symptoms are heavy periods and significant menstrual pain.
The heavy bleeding comes from increased blood vessel density in the affected areas and the growth of new, abnormal blood vessels within the uterine wall. The tissue embedded in the muscle also produces higher levels of nitric oxide, a molecule that dilates blood vessels and increases blood flow during menstruation. Together, these changes mean more blood reaches the uterine lining and more of it escapes during your period.
The pain side of adenomyosis has its own biology. The uterus becomes hypercontractile, squeezing harder and more often than normal due to increased oxytocin receptor activity. Inflammatory molecules like prostaglandins build up in the affected tissue, and nerve fibers in the uterine lining become sensitized. Women with painful adenomyosis have a measurably higher density of nerve fibers in their uterine tissue compared to those with the condition who don’t experience pain.
Endometrial Polyps
Polyps are small, soft growths that develop on the inner lining of the uterus. They’re usually benign and range from a few millimeters to several centimeters. Like fibroids, they increase the surface area of the lining and create fragile blood vessels that bleed easily. Polyps can cause heavy periods, bleeding between periods, or irregular spotting. They’re most common in women in their 40s and 50s and are typically detected with ultrasound.
Bleeding Disorders
An underlying bleeding disorder is a surprisingly common and frequently overlooked cause of heavy periods. Among women with confirmed heavy menstrual bleeding, an estimated 5% to 20% have von Willebrand disease, a condition where the blood doesn’t clot properly due to a deficiency in a specific clotting protein. Flipping that around, 60% to 95% of women who have von Willebrand disease report heavy periods as one of their most significant symptoms.
Heavy periods that started with your very first cycle, a history of easy bruising, prolonged bleeding after dental work or surgery, or a family history of bleeding problems all point toward this possibility. Platelet disorders and other clotting factor deficiencies can produce the same pattern. These conditions are often missed because heavy periods are normalized or attributed to hormonal causes without further investigation.
Thyroid Problems
An underactive thyroid can shift your body’s clotting system toward a state where blood doesn’t clot as effectively and existing clots break down faster than they should. The most common clotting problem seen in overt hypothyroidism is acquired von Willebrand syndrome, where low thyroid hormone levels reduce the production or release of the same clotting protein that’s deficient in the inherited form. Replacing thyroid hormone typically reverses this effect. Hypothyroidism can also cause other menstrual irregularities, including missed periods, lighter periods, or bleeding between cycles.
Copper IUD
The copper IUD is a highly effective, hormone-free contraceptive, but heavier periods are its most common side effect. In studies, women using a copper IUD saw their average menstrual blood loss increase from about 36 ml to roughly 40 to 54 ml per cycle, depending on the device type. The increase is most noticeable in the first two periods after insertion and tends to improve somewhat by the third cycle. For women who already had heavy periods before insertion, the copper IUD pushed average blood loss above 100 ml per cycle, well into the clinical range for heavy menstrual bleeding.
Other Contributing Factors
Several additional conditions can cause or worsen heavy periods. Endometriosis, where uterine-like tissue grows outside the uterus, is associated with heavier and more painful cycles. Certain medications, particularly blood thinners and anti-inflammatory drugs, can increase menstrual blood loss. Obesity raises estrogen levels because fat tissue converts other hormones into estrogen, contributing to the same kind of lining overgrowth seen in hormonal imbalance. In rare cases, cancers or precancers of the uterine lining cause heavy or irregular bleeding, which is why new-onset heavy bleeding after menopause always warrants prompt evaluation.
The Iron Connection
One of the most significant consequences of heavy periods is iron deficiency, which can develop long before full-blown anemia shows up on a blood test. In a study of women with von Willebrand disease, 66% reported heavy periods, and 45% had low iron stores even when their hemoglobin levels were still in the normal range. This matters because iron deficiency without anemia still causes fatigue, brain fog, hair loss, and exercise intolerance. If your periods are heavy, checking ferritin (your stored iron) is more informative than hemoglobin alone.
How the Cause Is Identified
Figuring out why your periods are heavy usually starts with a pelvic exam and a transvaginal ultrasound, which is the first-line imaging test for spotting fibroids, polyps, and signs of adenomyosis. If the ultrasound is inconclusive, a saline infusion sonography may follow. This involves filling the uterus with a small amount of saline during the ultrasound to get a clearer picture of the inner cavity. A normal result on this test effectively rules out structural problems inside the uterus. MRI is reserved for specific situations, like confirming adenomyosis or planning fibroid treatment.
Blood work typically includes hemoglobin and ferritin to assess for anemia and iron depletion. Testing for clotting disorders isn’t routine but should be considered if you’ve had heavy periods since your teens, bruise easily, or have a family history of bleeding problems. One practical tool used to quantify blood loss is a pictorial blood assessment chart, where you track the number and saturation level of pads or tampons used each day. A score above 150 on this chart correlates well with blood loss over 80 ml per cycle, giving both you and your doctor an objective measure to work from.