Heavy periods happen when something disrupts the normal balance of hormones, the structure of the uterus, or your body’s ability to form clots. Doctors define heavy menstrual bleeding as losing more than 80 milliliters (about 2.7 ounces) of blood per cycle, but a more practical measure is this: if you’re soaking through a pad or tampon in less than two hours, or passing large clots, your bleeding qualifies as heavy. There are several reasons this happens, and most are treatable once you know the cause.
How to Tell If Your Bleeding Is Actually Heavy
It’s hard to measure blood loss in milliliters at home, so the CDC uses a simpler guideline: if you’re soaking through one or more pads or tampons every hour for several hours in a row, that’s heavy bleeding. Other signs include periods lasting longer than seven days, needing to double up on protection, waking up at night to change pads, or feeling exhausted and short of breath (which can signal blood loss over time).
Many people assume their flow is normal because they’ve never known anything different. If your period has always been heavy, it’s still worth investigating. Some causes, like bleeding disorders, are present from your very first cycle but go undiagnosed for years.
Hormonal Imbalance and Thick Uterine Lining
The most common reason for heavy periods is a hormonal imbalance between estrogen and progesterone. In a normal cycle, estrogen builds up the uterine lining during the first half, and then progesterone stabilizes it after ovulation. When you don’t ovulate (which can happen more often than you’d think, especially during puberty, perimenopause, or with conditions like PCOS), progesterone never kicks in. Estrogen keeps thickening the lining unopposed, and when it finally sheds, there’s simply more tissue and more blood to lose.
This “unopposed estrogen” pattern can also cause a condition called endometrial hyperplasia, where the lining becomes abnormally thick. It’s one of the most common presentations of that overgrowth, and it’s the reason doctors pay close attention to heavy bleeding in people with obesity, PCOS, or irregular cycles. All of these conditions create an environment where estrogen dominates without progesterone’s balancing effect.
Fibroids
Uterine fibroids are noncancerous growths in the wall of the uterus, and they’re extremely common. They cause heavy bleeding through several overlapping mechanisms. As a fibroid grows, it increases the surface area of the uterine lining, giving more tissue to shed each month. Fibroids also compress surrounding blood vessels, creating enlarged, dilated veins that bleed more freely. The blood vessels that form around fibroids tend to be structurally fragile and chaotic, similar to the disorganized blood supply seen around tumors, making them prone to breaking and leaking.
Fibroids also interfere with the uterus’s ability to contract and clamp down on bleeding vessels after shedding its lining. On top of that, they release chemical signals that reduce your endometrium’s natural clotting ability, thinning out the local clot-forming response. The combination of more surface area, weaker vessels, impaired contraction, and reduced clotting explains why fibroids are one of the most reliable causes of truly heavy flow.
Adenomyosis
Adenomyosis is a condition where tissue that normally lines the inside of the uterus grows into the muscular wall itself. Each month, that misplaced tissue responds to your hormones just like the normal lining does: it thickens, breaks down, and bleeds. But because it’s trapped inside the muscle, it causes the uterus to enlarge and become boggy, leading to heavier and often more painful periods. Adenomyosis is frequently missed on standard exams and can coexist with fibroids, making it harder to pin down as the cause.
Bleeding Disorders
Between 5% and 24% of people with chronic heavy periods have an underlying bleeding disorder, most commonly von Willebrand disease. This condition affects the blood’s ability to clot properly, and heavy periods are often its earliest and most noticeable symptom. The prevalence varies by background: studies have found it in roughly 16% of white women with heavy bleeding compared to about 1% of Black women with the same complaint.
If your periods have been heavy since they first started, if you bruise easily, bleed a long time after dental work, or have a family history of bleeding problems, a clotting disorder is worth considering. It’s diagnosed with blood tests your doctor can order, and treatments exist that specifically target the clotting problem rather than just managing the flow.
Thyroid Problems
An underactive thyroid has a well-established link to heavy periods. In one study of 50 women with hypothyroidism, 36% reported heavy menstrual bleeding as their primary menstrual complaint. The connection runs deep enough that treating the thyroid condition with thyroid hormone replacement measurably reduces menstrual blood loss. If you’re also dealing with fatigue, weight gain, cold sensitivity, or dry skin alongside heavy periods, thyroid function is a straightforward thing to test for.
Polyps and Other Structural Causes
Endometrial polyps are small, soft growths that protrude from the uterine lining. They have their own blood supply and can bleed between periods or make periods heavier. Unlike fibroids, which grow in the muscle wall, polyps sit on the inner surface and are usually easier to remove. Other structural causes include a copper IUD, which commonly increases menstrual flow, and, rarely, precancerous or cancerous changes to the lining.
How Heavy Bleeding Is Diagnosed
Diagnosis typically starts with blood work to check for anemia, thyroid function, and clotting problems. A pelvic ultrasound can reveal fibroids, polyps, or an enlarged uterus. For a closer look at the uterine lining, your doctor may recommend a transvaginal ultrasound or a saline-infusion sonogram, where fluid is used to open the uterine cavity for a clearer image.
If you’re over 45, or younger with risk factors like obesity, PCOS, or bleeding that hasn’t responded to treatment, an endometrial biopsy is typically recommended. This is a brief in-office procedure that collects a small sample of the uterine lining to check for hyperplasia or abnormal cells. If that sample comes back inconclusive and bleeding persists, a hysteroscopy (where a small camera is inserted into the uterus) may be the next step.
How Heavy Periods Affect Your Health
The biggest downstream consequence of heavy periods is iron deficiency, which progresses to anemia when blood loss outpaces your body’s ability to replace red blood cells. Symptoms include fatigue, dizziness, shortness of breath with mild activity, brain fog, and pale skin. Many people attribute these symptoms to stress or poor sleep without connecting them to their period.
Iron deficiency can also cause problems before it reaches the level of full-blown anemia. Low iron stores without anemia still affect energy levels, exercise tolerance, and cognitive function. If your periods are heavy, checking your iron levels (specifically ferritin, which measures stored iron) gives a more complete picture than a standard blood count alone. Ferritin below 20 ng/mL with heavy bleeding is a clear signal your body’s reserves are depleted.
How Heavy Periods Are Treated
Treatment depends entirely on the cause. For hormonal imbalances, the first-line approach is hormonal therapy: birth control pills, a hormonal IUD, or cyclic progesterone. These work by thinning the uterine lining or regulating the hormonal cycle so less tissue builds up in the first place. A hormonal IUD is particularly effective because it delivers progesterone directly to the uterus and can reduce bleeding by up to 90% in many users.
For fibroids or polyps, the treatment path depends on size, location, and severity of symptoms. Small polyps can often be removed in an office procedure. Fibroids may be managed with medication, treated with minimally invasive procedures that cut off their blood supply, or surgically removed while preserving the uterus. For bleeding disorders, medications that support clot formation can significantly reduce flow.
A non-hormonal option is tranexamic acid, which helps blood clots stay intact and is taken only during your period. It’s particularly useful for people who can’t or prefer not to use hormonal treatments. For thyroid-related heavy bleeding, treating the thyroid condition itself typically brings periods back to a normal volume without any additional intervention.