Heavy periods happen when something disrupts the normal balance between building up and shedding your uterine lining each month. Clinically, heavy menstrual bleeding is defined as losing more than 80 milliliters of blood per cycle, though in practice most people recognize it by soaking through a pad or tampon every hour or two, passing large clots, or bleeding for more than seven days. The causes range from hormonal imbalances to structural growths in the uterus to underlying health conditions you might not connect to your period at all.
Hormonal Imbalance and Lining Overgrowth
Your menstrual cycle relies on a back-and-forth between two hormones. Estrogen thickens the uterine lining in the first half of your cycle, and progesterone stabilizes it after ovulation. If you don’t ovulate in a given month, progesterone never rises, and your lining keeps growing unchecked. When it finally sheds, there’s simply more tissue and more blood to lose.
This pattern of estrogen without adequate progesterone is one of the most common reasons for heavy periods. It can happen occasionally in anyone, but it’s especially frequent at two life stages: the first few years after periods begin (when the hormonal system is still maturing) and during perimenopause. The perimenopausal transition typically starts five to ten years before your final period, and during that window, ovulation becomes inconsistent. Cycles may stretch longer, then come early, and the bleeding itself often gets noticeably heavier because the lining has had extra time to build up before it sheds.
When this imbalance becomes chronic, it can lead to a condition called endometrial hyperplasia, where the lining grows abnormally thick and the cells become crowded and irregular. This is worth paying attention to because certain types of hyperplasia carry a small risk of progressing to uterine cancer, particularly in women over 45.
Fibroids, Polyps, and Other Structural Causes
Uterine fibroids are noncancerous growths in the muscular wall of the uterus, and they’re remarkably common. Fibroids make periods heavier through a few different mechanisms. They increase the total surface area of the uterine lining, meaning there’s more tissue to bleed from. They also interfere with the uterus’s ability to contract and clamp down on blood vessels after shedding, which is how your body normally slows bleeding. On top of that, fibroids create pressure that enlarges veins within the lining, forming what researchers describe as “venous lakes” that release more blood during your period.
Not all fibroids cause heavy bleeding. Location matters more than size. Fibroids that bulge into the uterine cavity (submucosal fibroids) are most likely to affect your flow, while ones growing on the outer surface of the uterus may cause no menstrual symptoms at all.
Endometrial polyps work differently but produce similar results. These are small, soft growths on the lining itself that have their own blood supply and can bleed between periods or make periods heavier. They’re usually benign but occasionally need to be removed for testing.
Adenomyosis: Lining Tissue in the Wrong Place
Adenomyosis happens when the cells that normally line the inside of the uterus start growing into the muscular wall. This causes the uterus to enlarge and become boggy and tender. Because those displaced cells still respond to your monthly hormonal cycle, they swell and bleed within the muscle itself, leading to heavy periods and significant cramping.
Adenomyosis is an estrogen-dependent condition, meaning it tends to worsen during the reproductive years and improve after menopause. It’s notoriously underdiagnosed because the main symptoms, heavy bleeding and pelvic pain, overlap with so many other conditions. MRI is often the most accurate way to confirm it.
Thyroid Problems and Bleeding Disorders
Your thyroid might seem unrelated to your period, but low thyroid hormone levels (hypothyroidism) shift your body’s clotting system in a direction that makes bleeding harder to stop. Specifically, hypothyroidism reduces levels of key clotting proteins, including von Willebrand factor and several other coagulation factors, while also increasing the rate at which your body breaks down clots. The result is heavier, longer periods. Treating the thyroid problem typically reverses the bleeding issue.
Inherited bleeding disorders are another overlooked cause. Von Willebrand disease, the most common inherited bleeding disorder, is found in an estimated 5 to 20 percent of women who report heavy menstrual bleeding. Many of these women go years without a diagnosis because they assume their periods are just “normal for them.” Clues that a bleeding disorder might be involved include heavy periods starting from your very first cycle, easy bruising, prolonged bleeding after dental work or minor cuts, and a family history of bleeding problems.
Copper IUDs
If your periods became heavier after getting a copper IUD, you’re not imagining it. Research measuring used sanitary products shows that copper IUDs increase menstrual blood loss by about 50 percent over pre-insertion levels. Unlike many side effects that ease with time, this increase appears to remain constant for at least the first 12 months. Hormonal IUDs, by contrast, typically make periods lighter.
How Heavy Bleeding Affects Your Body
The biggest downstream consequence of chronically heavy periods is iron deficiency. Every period costs you iron, and when the losses outpace what you absorb from food, your iron stores drop. The World Health Organization defines iron deficiency in women as a ferritin level below 30 ng/mL and anemia as hemoglobin below 12 g/dL. Symptoms creep in gradually: fatigue that doesn’t improve with sleep, brain fog, shortness of breath on stairs, hair thinning, restless legs, and feeling cold all the time. Many women attribute these symptoms to stress or aging rather than connecting them to their periods.
How the Cause Is Identified
Figuring out why your periods are heavy usually starts with a standard pelvic exam and blood work to check your blood count, iron levels, and thyroid function. The most important imaging step is a transvaginal ultrasound, which can reveal fibroids, polyps, and signs of adenomyosis. If the ultrasound is inconclusive, a saline infusion sonography (where a small amount of sterile fluid is used to expand the uterine cavity during the scan) gives a clearer picture of anything growing inside the cavity.
Hysteroscopy, where a thin camera is passed through the cervix to look directly at the lining, is sometimes used when imaging isn’t definitive or when a growth needs to be biopsied or removed at the same time. For women 45 and older, or when initial treatments aren’t working, an endometrial biopsy is typically recommended to rule out precancerous changes in the lining. MRI is reserved for specific situations like confirming adenomyosis or planning treatment for large fibroids.
Because the causes of heavy periods range from a simple hormonal blip to a structural problem to a systemic condition like a thyroid or bleeding disorder, there isn’t a single test that covers everything. The process is usually stepwise: start with the most common and least invasive investigations, then move to more targeted ones based on what turns up.