Heavy periods happen when something disrupts the normal process of building and shedding the uterine lining each month. Clinically, a period is considered heavy when you lose more than 80 milliliters of blood per cycle, roughly equivalent to soaking through a pad or tampon every hour or two for several consecutive hours. The causes range from hormonal shifts and structural changes in the uterus to bleeding disorders and medications, and sometimes several factors overlap at once.
How to Tell If Your Period Is Actually Heavy
It can be hard to measure blood loss directly, so practical signs matter more than milliliters. You likely have heavy menstrual bleeding if you’re soaking through a pad or tampon every one to two hours, passing blood clots the size of a quarter or larger more than once or twice per period, or bleeding for longer than seven days. Needing to double up on protection (a pad plus a tampon), waking up at night to change products, or feeling exhausted and lightheaded during your period are also strong signals.
Heavy bleeding isn’t just an inconvenience. Over time, it drains your iron stores and can lead to iron deficiency anemia, which shows up as fatigue, brain fog, shortness of breath, and pale skin. A hemoglobin level below 12 g/dL confirms anemia, but ferritin (your body’s stored iron) is an even earlier warning sign. Many experts flag a ferritin level below 30 as iron deficient, and some use a threshold of 50 for people with ongoing blood loss.
Hormonal Imbalances and Ovulation Problems
The most common reason for heavy periods, especially ones that come irregularly, is a hormone imbalance between estrogen and progesterone. In a typical cycle, estrogen thickens the uterine lining during the first half, and progesterone stabilizes it after ovulation. When you don’t ovulate (a cycle called anovulatory), progesterone never kicks in. Estrogen keeps building the lining unopposed, making it much thicker than normal. When it finally sheds, there’s simply more tissue and more blood to lose.
Conditions that interfere with ovulation are a major driver of this pattern. Polycystic ovary syndrome (PCOS) is one of the most common, but thyroid disorders (both overactive and underactive) and high stress levels that affect the brain’s hormonal signaling can also cause it. This type of heavy bleeding is especially common at two ends of reproductive life: in teenagers whose cycles haven’t fully regulated yet, and in people approaching menopause whose ovulation becomes less consistent.
Fibroids
Uterine fibroids are benign growths of smooth muscle in the uterine wall, and they’re extremely common. By age 50, the majority of women will have at least one. Many fibroids cause no symptoms at all, but those that grow near or into the inner lining of the uterus (called submucosal fibroids) can cause significantly heavier and longer periods. They increase the surface area of the lining, distort the uterine cavity, and can interfere with the uterus’s ability to contract and clamp down on bleeding vessels after shedding.
Adenomyosis
Adenomyosis is a condition where tissue that normally lines the inside of the uterus grows into the muscular wall instead. This leads to a swollen, enlarged uterus that bleeds more heavily and painfully. The embedded tissue triggers chronic inflammation, which ramps up the production of inflammatory chemicals called prostaglandins. Those prostaglandins cause intense cramping and also promote the growth of new blood vessels within the uterine wall, increasing the amount of blood present during a period.
On top of that, adenomyosis makes the uterine muscle hypercontract. The muscle cells in an affected uterus are more responsive to contraction signals, which contributes to severe menstrual pain and pain during sex. Adenomyosis is frequently underdiagnosed because its symptoms overlap with fibroids, but it tends to cause both heavy bleeding and significant pain together.
Polyps
Endometrial polyps are small, finger-like growths on the uterine lining. They’re usually benign, but they create extra surface area that bleeds during your period and can also cause spotting between periods. Polyps are more common after age 40 and in people with obesity or high blood pressure, but they can develop at any age. They’re one of the more treatable causes of heavy bleeding since they can typically be removed in a straightforward procedure.
Bleeding Disorders
Not all causes of heavy periods originate in the uterus. Among people with chronic heavy menstrual bleeding, somewhere between 5% and 24% have an underlying bleeding disorder. The most common one is von Willebrand disease, a condition where the blood doesn’t clot properly. It’s particularly worth considering in teenagers who have had heavy periods from their very first cycle, or in anyone with a history of easy bruising, frequent nosebleeds, or prolonged bleeding after dental work or injuries.
Von Willebrand disease appears to be more prevalent among white women (about 16% of those with heavy bleeding) compared to Black women (roughly 1%), though testing is recommended regardless of background when symptoms are suggestive. Because bleeding disorders are often missed as a diagnosis, many people go years assuming their heavy periods are just “normal for them.”
Medications and IUDs
Certain medications can make periods heavier as a side effect. Blood thinners are the most obvious culprit, but some hormonal treatments can also disrupt bleeding patterns. One of the most common and frequently surprising causes is the copper IUD. Because it’s hormone-free, many people choose it without expecting changes to their flow, but studies show blood loss increases by about 40% to 60% in the first couple of months after insertion. In one study, average blood loss went from about 37 ml per period before the copper IUD to around 54 ml afterward. This increase tends to lessen somewhat over time but often remains above the person’s baseline.
Hormonal IUDs, by contrast, typically reduce menstrual bleeding significantly and are actually used as a treatment for heavy periods. So the type of IUD matters enormously if flow is a concern.
Endometrial and Structural Problems
Sometimes the lining of the uterus itself doesn’t function properly even without an obvious structural growth. The endometrium has its own system for controlling blood vessel constriction and clotting when it sheds. If that local control system is disrupted, whether from chronic inflammation, infection, or abnormalities in how blood vessels tighten, bleeding can be heavier or more prolonged than expected. This is one of the harder causes to pin down because it doesn’t show up on imaging the way fibroids or polyps do.
In rare cases, heavy bleeding signals something more serious like endometrial hyperplasia (a precancerous thickening of the lining) or endometrial cancer. Risk factors include prolonged exposure to estrogen without progesterone, which can happen with obesity, PCOS, or certain medications. Irregular heavy bleeding that develops after years of normal periods, particularly after age 45, warrants evaluation.
Why Heavy Bleeding Often Has Multiple Causes
One reason heavy periods can be frustrating to sort out is that causes frequently stack. You might have a small fibroid that on its own would cause mildly heavier flow, combined with inconsistent ovulation from thyroid problems, combined with a copper IUD. Each factor adds to the total. This is also why treatment sometimes involves addressing more than one thing at once.
The diagnostic process typically starts with blood work to check for anemia, iron levels, thyroid function, and sometimes clotting factors. An ultrasound can identify fibroids, polyps, and signs of adenomyosis. If results are inconclusive, a closer look at the uterine cavity with a small camera (hysteroscopy) or a biopsy of the lining may follow. Treatment depends entirely on the underlying cause, ranging from hormonal options that thin the lining and regulate cycles, to medications that help blood clot more effectively during your period, to procedures that remove fibroids or polyps when they’re the main driver.