Blood clots during your period are usually just a normal part of menstruation. Your body sheds the lining of your uterus each cycle, and when the flow is heavy, blood can pool and clot before leaving your body. Small clots, roughly the size of a dime or quarter, are common and typically nothing to worry about. But when clots are larger, more frequent, or accompanied by unusually heavy bleeding, something else may be going on.
How Period Clots Form
During your period, your uterus sheds its inner lining along with blood. Your body normally releases natural anticoagulants to keep menstrual blood fluid as it leaves. When bleeding is heavy or fast, those anticoagulants can’t keep up, and the blood begins to clot before it exits. This is why clots tend to show up on your heaviest days, usually day one or two of your period.
The color and texture of clots can vary. Bright red clots are fresh, while darker, jelly-like clots have spent more time in the uterus before passing. Both are normal in small amounts. What matters more than color is size and frequency.
Normal Clots vs. Concerning Ones
Clots up to about the size of a quarter are considered normal for most people. It becomes a problem when you’re passing golf ball-sized clots, especially if that’s happening every couple of hours. The CDC flags clots the size of a quarter or larger as one sign of heavy menstrual bleeding, also called menorrhagia. If you’re soaking through a pad or tampon every hour for several consecutive hours, that’s another red flag that your bleeding has crossed into abnormal territory.
Hormonal Imbalances
One of the most common reasons for heavy periods with clots is an imbalance between estrogen and progesterone. Estrogen builds up the uterine lining each month, and progesterone helps regulate that growth and triggers shedding. When estrogen levels are disproportionately high relative to progesterone, the lining grows thicker than it should. A thicker lining means more tissue and blood to shed, which leads to heavier flow and more clotting.
This imbalance can happen at various life stages. It’s especially common during the teenage years when cycles are still establishing a pattern, and again during perimenopause when hormone levels fluctuate unpredictably. Conditions like polycystic ovary syndrome (PCOS) and obesity can also tip the balance toward excess estrogen. In some cases, this persistent thickening is diagnosed as endometrial hyperplasia, a condition where the uterine lining doesn’t fully shed each month and continues to build up. Synthetic progesterone therapy is often used to counteract this effect and bring the lining back to a normal thickness.
Uterine Fibroids and Polyps
Fibroids are noncancerous growths in or on the wall of the uterus. They’re extremely common, affecting up to 80% of women by age 50, and they’re one of the leading causes of heavy, clot-filled periods. Fibroids can distort the uterine cavity, increase the surface area of the lining, and interfere with the uterus’s ability to contract and stop bleeding efficiently. Not all fibroids cause symptoms, but those that grow into the uterine cavity tend to produce the heaviest bleeding.
Polyps are smaller, finger-like growths on the uterine lining. They can cause irregular bleeding and heavier flow for similar reasons: they add extra tissue that bleeds during your period.
Adenomyosis
Adenomyosis occurs when the tissue that normally lines the uterus starts growing into the muscular wall of the uterus itself. This causes the uterus to thicken and enlarge, sometimes to double or triple its usual size. The result is painful periods with heavy, prolonged bleeding and clotting. Researchers aren’t entirely sure what triggers it, but hormones, genetics, inflammation, and prior uterine trauma (like a cesarean section) may all play a role. It’s most commonly diagnosed in women in their 30s and 40s.
Bleeding Disorders
Sometimes the issue isn’t in the uterus at all. Von Willebrand disease is the most common inherited bleeding disorder in women, affecting roughly 1 in 100. But among women who report chronic heavy menstrual bleeding, the prevalence jumps dramatically, ranging from 5% to 24%. Among those with a confirmed diagnosis of Von Willebrand disease, 74% to 92% experience heavy menstrual bleeding as one of their primary symptoms.
This disorder affects how well your blood clots throughout your body, not just during your period. If you’ve always had heavy periods with large clots, bruise easily, bleed heavily after dental work, or have a family history of bleeding problems, a blood-clotting disorder is worth investigating. A simple blood test can check for it.
Early Pregnancy Loss
An unusually heavy “period” with large clots can sometimes be an early miscarriage, particularly if the bleeding is heavier than your normal cycle and the cramping is more intense than what you typically experience. This can happen before you even realize you’re pregnant, especially in the first few weeks. Other signs include the sudden loss of pregnancy symptoms like breast tenderness or nausea. If you have very heavy bleeding (soaking through at least two pads in an hour) and feel unwell, that warrants emergency care.
How Clotting Causes Are Identified
When you bring up heavy clotting with a doctor, they’ll typically start with a few key steps. Blood tests check for iron deficiency anemia (a common consequence of heavy periods) along with thyroid disorders and clotting problems. An ultrasound uses sound waves to create images of your uterus and ovaries, which can reveal fibroids, polyps, or signs of adenomyosis. In some cases, a sonohysterography adds fluid to the uterus during the ultrasound to get a clearer picture of the lining. A hysteroscopy involves a thin, lighted instrument that lets a doctor look directly inside the uterus for growths or abnormalities.
The Anemia Connection
Losing a lot of blood every month takes a toll. Heavy periods are one of the leading causes of iron deficiency anemia in women, and many people don’t connect their symptoms to their cycle. If you’re experiencing extreme tiredness, weakness, pale skin, dizziness, cold hands and feet, brittle nails, shortness of breath, or a fast heartbeat, chronic blood loss from heavy periods could be the reason. Some people with iron deficiency anemia develop unusual cravings for ice, dirt, or other non-food items, a condition called pica.
These symptoms tend to creep in gradually, so you may not notice how much your energy has dropped until you’re significantly depleted. If your periods regularly involve large clots and heavy flow, it’s worth having your iron levels checked even if you feel “mostly fine.” Low iron is treatable, and addressing it can make a noticeable difference in how you feel day to day.