Heavy periods happen when something causes the uterine lining to grow too thick, when the uterus can’t contract properly to stop bleeding, or when a clotting problem prevents blood from slowing down on its own. Clinically, a period is considered heavy when you lose more than 80 mL of blood per cycle or bleed for longer than seven days. In practical terms, if you’re soaking through a pad or tampon every one to two hours, passing clots the size of a quarter or larger, or doubling up on protection just to get through the day, your flow is heavier than normal.
Hormonal Imbalance and Lining Buildup
The most common driver 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 thickens the uterine lining to prepare for a potential pregnancy. After ovulation, progesterone stabilizes that lining and, if no pregnancy occurs, drops to trigger your period. When this system works correctly, the lining sheds in an orderly way over a few days.
Problems start when estrogen runs high relative to progesterone. This can happen if you skip ovulation in a given cycle, since ovulation is what triggers progesterone production in the first place. Without enough progesterone to keep it in check, estrogen continues thickening the uterine lining beyond what’s normal. When that overgrown lining finally sheds, there’s simply more tissue and more blood to pass, resulting in a heavier, longer period. Over time, persistent estrogen dominance can lead to a condition called endometrial hyperplasia, where the lining becomes abnormally thick.
Fibroids and Adenomyosis
Uterine fibroids are noncancerous growths in or on the uterus, and they’re one of the most well-known structural causes of heavy bleeding. Fibroids that grow into the uterine cavity or within the muscular wall can distort the lining, increase its surface area, and interfere with the uterus’s ability to contract and clamp down on blood vessels after your period starts. The result is heavier flow and sometimes prolonged bleeding.
Adenomyosis is a related but distinct condition. It occurs when the tissue that normally lines the inside of the uterus grows into the muscular wall itself. During your period, that embedded tissue also thickens, breaks down, and bleeds, just like the lining inside the uterus. This makes the uterus enlarge over time and produces noticeably heavier, more painful periods. Adenomyosis often coexists with fibroids and endometriosis, which can make it harder to identify as the specific source of heavy bleeding.
Thyroid Problems and PCOS
Your thyroid gland plays a surprisingly large role in menstrual flow. Hypothyroidism, where the thyroid is underactive, disrupts your cycle through several pathways at once. Low thyroid hormones trigger the brain to overproduce signals trying to stimulate the thyroid. This overproduction can spill over and interfere with the hormones that control ovulation, leading to elevated estrogen that goes unopposed by progesterone. The end result is the same excessive lining buildup described above.
Hypothyroidism also affects clotting. It can cause platelet dysfunction and even an acquired form of von Willebrand syndrome, a condition where the blood doesn’t clot efficiently. So an underactive thyroid can hit you twice: it builds a thicker lining and makes it harder for bleeding to stop once it starts. Even subclinical hypothyroidism, where lab values are only mildly off, can contribute to heavier periods and anemia.
Polycystic ovary syndrome (PCOS) leads to heavy periods through a different but overlapping route. PCOS frequently causes irregular or absent ovulation, which means progesterone stays low for extended stretches. Estrogen continues to build the uterine lining cycle after cycle without a proper shed. When a period finally does arrive, the accumulated lining can produce an episode of very heavy, prolonged bleeding.
Bleeding Disorders
Not all heavy periods come from the uterus itself. Between 5% and 24% of people with chronic heavy menstrual bleeding have an underlying bleeding disorder, with von Willebrand disease being the most common. This condition affects the blood’s ability to form clots, and for many people, heavy periods are the first and most obvious symptom. Prevalence varies by ethnicity: roughly 16% among white women with heavy bleeding compared to about 1% among Black women.
Bleeding disorders are frequently overlooked because heavy periods are often dismissed as normal variation. If your periods have been heavy since your very first cycle, or if you also bruise easily, bleed heavily after dental work, or have a family history of bleeding problems, a clotting issue is worth investigating.
IUDs and Medications
The type of contraception you use can dramatically shift your flow in either direction. A copper IUD increases menstrual blood loss by roughly 50%, and heavier, longer periods are one of the most common reasons people have it removed. The copper triggers an inflammatory response inside the uterus that leads to more bleeding during each cycle.
Hormonal IUDs work in the opposite direction. The most well-studied hormonal IUD reduces menstrual bleeding by up to 90% within six months, making it one of the most effective treatments for heavy periods as well as a contraceptive. If your periods became noticeably heavier after getting a copper IUD, the device itself is likely the cause.
Blood thinners, including aspirin taken regularly, can also increase menstrual flow by interfering with the clotting process that normally helps your period taper off. Some anti-inflammatory medications and certain herbal supplements with blood-thinning properties can have a similar effect.
Perimenopause and Hormonal Shifts
If your periods suddenly became heavier in your late 30s or 40s, perimenopause is a likely explanation. During this transition, the brain ramps up its signals to the ovaries, trying to compensate for declining ovarian function. This “flogging” of the ovaries, as reproductive endocrinologists describe it, can actually push estrogen levels higher than they were during peak reproductive years. The result is a thicker uterine lining and heavier bleeding, sometimes described as “flooding” because of how suddenly and intensely it can come on.
These episodes are unpredictable. You might have a light period one month and a dramatically heavy one the next, because ovulation becomes erratic. Cycles where you don’t ovulate produce no progesterone, so the lining just keeps growing until it outstrips its own blood supply and sheds in a heavy, disorganized way. This pattern can continue for several years before periods stop entirely at menopause.
How Heavy Periods Affect Your Health
The biggest downstream risk of persistently heavy periods is iron deficiency anemia. Every heavy cycle depletes your iron stores, and for many people, diet alone can’t keep up with the losses. Iron deficiency shows up as fatigue, brain fog, dizziness, shortness of breath during exercise, and pale skin. It’s diagnosed when ferritin levels drop below 30 μg/L, though symptoms can appear even before levels fall that low.
Iron deficiency from heavy periods is extremely common and frequently undertreated, particularly in teenagers and young adults who may not realize their bleeding is abnormal because they have no baseline for comparison. Fatigue that doesn’t improve with more sleep, difficulty concentrating at school or work, and feeling winded climbing stairs are all signs that heavy periods may be draining your iron faster than you can replace it.