A heavy period is most often caused by a hormonal imbalance between estrogen and progesterone, but it can also result from structural changes in the uterus, thyroid problems, or an underlying bleeding disorder. A typical period lasts four to five days and produces about two to three tablespoons of blood. Periods that last longer than seven days or produce roughly twice that volume are considered heavy.
Hormonal Imbalance: The Most Common Cause
Your menstrual cycle depends on a careful balance between two hormones: estrogen and progesterone. Estrogen builds up the uterine lining each month, and progesterone keeps that growth in check. When progesterone levels are too low relative to estrogen, the lining grows thicker than it should. When it finally sheds, there’s simply more tissue and blood to pass, which translates to a heavier, longer period.
This imbalance can happen for a number of reasons. Cycles where you don’t ovulate (called anovulatory cycles) are one of the most common. Without ovulation, the ovary doesn’t produce the burst of progesterone that normally follows it, so estrogen goes essentially unopposed for weeks. The result is an irregularly thick lining that often sheds unevenly, causing prolonged or heavy bleeding. Anovulatory cycles are especially common at the bookends of reproductive life: during the first few years of menstruation and again during perimenopause.
Perimenopause and Changing Cycles
Starting in the late 30s, progesterone production naturally declines. The number and quality of ovarian follicles drop as well, leading to fewer ovulations and shifts in estrogen output. With less progesterone to regulate the uterine lining, it can build up significantly before it’s finally shed. Harvard Health describes this as one of the hallmarks of the perimenopausal transition, and it’s a major reason many people in their 40s experience periods that are suddenly heavier or less predictable than anything they dealt with in their 20s.
Estrogen-fueled conditions like fibroids and endometriosis can also worsen during this stage, compounding the bleeding. Perimenopause can last anywhere from a few years to over a decade, so these changes aren’t always a brief phase.
Fibroids and Adenomyosis
Uterine fibroids are noncancerous growths in the wall of the uterus. They’re extremely common, and depending on their size and location, they can distort the uterine cavity and increase the surface area of the lining that bleeds each month. Fibroids that grow just beneath the inner lining (submucosal fibroids) are the type most strongly linked to heavy periods.
Adenomyosis is a related but distinct condition. Instead of a growth sitting in the uterine wall, the tissue that normally lines the uterus migrates into the muscular wall itself. This causes the uterus to thicken and enlarge. The result is often heavy periods, clots during menstruation, and sometimes bleeding between periods. Over time, the blood loss can lead to fatigue, shortness of breath, and other signs of anemia. Adenomyosis is typically diagnosed with ultrasound or MRI, and it tends to affect people in their 30s and 40s.
Endometriosis, where uterine-like tissue grows outside the uterus on structures like the ovaries, bladder, or intestines, is better known for causing pain than heavy bleeding. But it can still contribute to heavier flow, particularly when it coexists with adenomyosis.
Thyroid Problems
An underactive thyroid (hypothyroidism) can cause heavy periods through several overlapping pathways. Low thyroid hormone disrupts the body’s clotting ability, sometimes triggering a form of acquired platelet dysfunction similar to von Willebrand disease. It also interferes with the hormonal signals between the brain and the ovaries. When thyroid hormone is low, the pituitary gland ramps up its output to compensate, and this ripple effect can suppress the hormones that trigger ovulation. Without ovulation, estrogen goes unopposed, the lining thickens excessively, and bleeding becomes heavy and irregular.
Even mildly low thyroid function (subclinical hypothyroidism) can contribute to heavier periods. If your periods have become noticeably heavier alongside symptoms like fatigue, weight gain, cold sensitivity, or constipation, a simple blood test can check your thyroid levels.
Bleeding Disorders
Some people have heavy periods not because of a uterine or hormonal problem, but because their blood doesn’t clot efficiently. Von Willebrand disease is the most common inherited bleeding disorder, and it disproportionately affects menstrual flow. Among people with chronically heavy periods, somewhere between 5% and 24% turn out to have von Willebrand disease, according to the American College of Obstetricians and Gynecologists. That’s a surprisingly high range, and it suggests the condition is significantly underdiagnosed.
Clues that a bleeding disorder might be involved include heavy periods that started with your very first cycle, frequent nosebleeds, easy bruising, or prolonged bleeding after dental work or minor cuts. A family history of bleeding problems is another important signal. Platelet function disorders and other clotting factor deficiencies can produce the same pattern.
Polyps and Other Structural Causes
Uterine polyps are small, usually benign growths that develop on the inner lining of the uterus. They can cause heavy periods, irregular spotting, or bleeding between cycles. Polyps are more common after age 40 but can occur at any age. An intrauterine device (IUD) that doesn’t release hormones, specifically the copper IUD, is another well-known cause of heavier flow, particularly in the first several months after insertion.
Infections and Inflammation
Pelvic inflammatory disease (PID) is an infection of the uterus, fallopian tubes, or ovaries, most commonly caused by gonorrhea or chlamydia. While PID is more closely associated with pain, fever, and unusual discharge, it can also cause bleeding between periods and disrupt normal menstrual patterns. Chronic inflammation in the reproductive tract can make the uterine lining more fragile and prone to irregular shedding.
How Heavy Bleeding Affects Your Body
The biggest downstream risk of persistently heavy periods is iron deficiency anemia. Every period depletes your iron stores to some degree, but when blood loss is excessive month after month, your body can’t replenish what it loses. A low ferritin level (your body’s stored iron), a low hemoglobin level, or both confirm the diagnosis. The symptoms are often so gradual that people don’t connect them to their periods: constant tiredness, weakness, dizziness, shortness of breath during everyday activities, pale skin, and difficulty concentrating.
If you’re soaking through a pad or tampon every hour for several consecutive hours, passing clots larger than a quarter, needing to double up on protection, or waking up at night to change pads, those are practical signs your flow has crossed into heavy territory. Tracking how often you change products and whether you’re seeing clots gives you concrete information to bring to a healthcare provider, which makes it much easier to figure out what’s driving the bleeding and what to do about it.