A period that suddenly becomes heavier than normal usually signals a shift in your hormones, a structural change in your uterus, or sometimes an underlying health condition you haven’t been diagnosed with yet. Clinically, bleeding is considered heavy when you lose more than 80 milliliters per cycle, but you don’t need to measure that precisely. If you’re soaking through a pad or tampon every hour or two, passing clots the size of a quarter or larger, or bleeding for more than seven days, your flow has crossed the line from normal variation into something worth investigating.
Hormonal Shifts Are the Most Common Cause
Your period is ultimately controlled by two hormones: estrogen and progesterone. Estrogen builds up the uterine lining each month, making it thicker and more blood-rich. Progesterone counteracts that growth and then, when it drops at the end of your cycle, triggers the lining to shed in an orderly way. When these two hormones fall out of balance, the system breaks down.
The most frequent scenario is a cycle where you don’t ovulate. Without ovulation, your body never produces the progesterone surge that normally keeps the lining in check. Estrogen continues building the lining unopposed, making it much thicker than usual. When it finally sheds, the bleeding is heavier, less organized, and often longer. This can happen during times of stress, significant weight change, or illness, and it can feel like it came out of nowhere.
Polycystic ovary syndrome (PCOS) is one of the most common reasons for these anovulatory cycles. In PCOS, excess androgens get converted into a form of estrogen in body fat, creating prolonged estrogen stimulation without progesterone to balance it. The lining grows excessively, and the body lacks the normal hormonal trigger to shed it cleanly. About 20% of people with PCOS and irregular periods develop endometrial thickening as a result.
Perimenopause Can Start Earlier Than You Think
If you’re in your late 30s or 40s and your periods have become unpredictably heavy, perimenopause is a likely explanation. This transition phase can begin up to a decade before menopause, and one of its hallmark signs is erratic bleeding. Estrogen rises and falls unpredictably, ovulation becomes inconsistent, and the time between periods stretches or shortens. Some cycles produce a normal flow while others are dramatically heavier. This isn’t a single event but a pattern that can continue for years as your body moves toward menopause.
Fibroids and Polyps
Uterine fibroids are noncancerous growths in the muscular wall of the uterus, and they’re extremely common. The ones most likely to cause heavy bleeding are intramural fibroids, which grow within the uterine wall. These fibroids develop their own blood supply, surrounded by a capsule of irregular, fragile blood vessels that form in a chaotic pattern similar to blood vessel growth in tumors. These structurally deficient vessels are prone to breaking and leaking, which adds significant volume to your menstrual flow.
Fibroids can also physically compress nearby veins, creating enlarged pools of blood within the uterine lining. Both of these mechanisms explain why a fibroid that’s been quietly growing for months or years can suddenly tip your period from manageable to heavy. Uterine polyps, which are smaller growths on the lining itself, can cause similar problems, though the mechanism is more straightforward: they create extra surface area that bleeds.
Your Thyroid May Be Involved
The thyroid gland helps regulate your menstrual cycle, and an underactive thyroid (hypothyroidism) is a well-established cause of heavier periods. The tricky part is that hypothyroidism develops slowly, often over years. You might notice fatigue or sluggishness first and not connect it to your heavier periods until much later. If your heavy bleeding came on gradually alongside symptoms like weight gain, feeling cold, or brain fog, a simple blood test can check your thyroid function.
Bleeding Disorders That Go Undiagnosed
This is one of the most overlooked causes of heavy periods, especially in younger people. Von Willebrand disease is an inherited condition that affects the blood’s ability to clot. It’s far more common than most people realize: among young patients who seek help for heavy periods, inherited bleeding disorders are present in roughly two-thirds of cases, and about a quarter of those involve von Willebrand disease specifically.
Many people with this condition go years without a diagnosis because heavy periods are often dismissed as “just how it is.” If your periods have always been heavy, or if you also bruise easily, bleed a long time after cuts or dental work, or have a family history of bleeding problems, this is worth raising with your doctor.
Copper IUDs and Heavy Bleeding
If you recently had a copper IUD placed, that alone can explain the change. Copper IUDs increase menstrual blood loss by about 50% over pre-insertion levels, and this increase appears to stay constant for at least the first year. In one study, about 68% of users reported heavier bleeding in the first nine weeks. That number dropped to about 49% by nine months, so there is some improvement over time, but the bleeding rarely returns to pre-IUD levels. Hormonal IUDs, by contrast, typically reduce menstrual flow significantly.
The Risk of Iron Deficiency
Consistently heavy periods drain your iron stores, and this can happen faster than you’d expect. Iron deficiency doesn’t always show up as full-blown anemia at first. You may feel exhausted, short of breath during exercise, or unable to concentrate well before your bloodwork looks abnormal. The World Health Organization considers iron stores depleted when ferritin (the protein that stores iron) falls below 15 ng/mL, and hemoglobin below 120 g/L indicates anemia. If your periods have been heavy for several months, checking your iron levels is a practical first step even before the underlying cause is identified.
How the Cause Gets Identified
The first step is usually a transvaginal ultrasound, which can reveal fibroids, polyps, or unusual thickening of the uterine lining. This works well for most people, but in more than a third of cases, the view of the uterine cavity is suboptimal, particularly when fibroids distort the anatomy. In those situations, additional imaging techniques like saline-infusion sonography, which fills the uterus with a small amount of fluid to improve the picture, can give a clearer answer. For people over 45, or when the lining appears unusually thick, an endometrial biopsy may be recommended to rule out precancerous changes.
Blood work typically includes a complete blood count to check for anemia, thyroid function tests, and sometimes hormone levels. If a bleeding disorder is suspected, specific tests for von Willebrand factor can confirm or rule it out.
Treatment Depends on the Cause
For hormonal imbalances and anovulatory cycles, hormonal treatments that provide progesterone (either through a hormonal IUD, oral medication, or combination birth control) can thin the lining and restore a more normal flow. A hormonal IUD is one of the most effective options, reducing blood loss more than any oral medication.
For bleeding that needs to be managed cycle by cycle, a medication that helps blood clot more effectively can reduce flow by 26% to 60%, depending on the dose. It’s taken only during the days of your period and doesn’t contain hormones. Fibroids and polyps, if they’re causing the problem, can often be removed through minimally invasive procedures.
If your flow has changed enough that you’re soaking through protection every hour for several consecutive hours, or if you’re passing large clots regularly, don’t wait to bring it up. Heavy bleeding has identifiable causes, and most of them are very treatable once you know what you’re dealing with.