Menstrual blood clots form when your flow is heavy enough that your body’s natural blood-thinning system can’t keep up. Small clots, roughly the size of a dime or smaller, are common and generally not a sign of anything wrong. But if you’re passing large or frequent clots, something is likely causing your uterine lining to build up thicker than normal or your body to shed it faster than usual. Several conditions can drive this, and most are treatable once identified.
How Clots Form During Your Period
During menstruation, your body releases anticoagulants, proteins that keep menstrual blood liquid so it can flow out easily. When bleeding is light or moderate, these anticoagulants do their job and you see little to no clotting. But when the flow is heavy, blood pools in the uterus or vagina faster than the anticoagulants can work. That pooled blood begins to coagulate, forming the jelly-like clots you see on your pad or in the toilet.
The thickness of your uterine lining directly affects how heavy the bleeding is. A thicker lining means more tissue and blood vessels to shed, which means a heavier flow and more opportunity for clots to form. Anything that causes that lining to grow excessively, or that disrupts normal shedding, can lead to noticeable clotting.
Hormonal Imbalance Is the Most Common Cause
In a normal cycle, estrogen builds up the uterine lining during the first half, and progesterone stabilizes it in the second half after ovulation. When these two hormones fall out of balance, particularly when estrogen runs high relative to progesterone, the lining grows thicker than it should. That extra-thick lining sheds as a heavier, clottier period.
One common way this happens is through anovulation, a cycle where your ovaries don’t release an egg. Without ovulation, your body doesn’t produce the usual surge of progesterone. Estrogen continues building the lining unopposed, sometimes for weeks longer than normal. When that lining finally sheds, the result is often a noticeably heavier period with more clots. Anovulatory cycles are especially common during puberty, the years leading up to menopause (perimenopause), after stopping hormonal birth control, and in people with polycystic ovary syndrome (PCOS).
Thyroid disorders can also throw off this balance. Both an underactive and overactive thyroid affect how your body produces reproductive hormones, and heavy, clotty periods are a well-known symptom of thyroid dysfunction.
Fibroids, Polyps, and Adenomyosis
Structural changes inside the uterus are another major driver of heavy clotting. These are physical growths or tissue changes that increase the surface area of the lining, distort the uterine cavity, or interfere with the uterus’s ability to contract and stop bleeding.
Uterine fibroids are noncancerous muscular growths in the uterine wall. They’re extremely common, affecting up to 80% of women by age 50. Fibroids that grow into the uterine cavity or sit within the wall can increase menstrual bleeding significantly. The larger the fibroid and the closer it is to the lining, the heavier the flow tends to be.
Uterine polyps are softer growths that form when cells in the uterine lining overgrow. They’re estrogen-sensitive, meaning they grow in response to estrogen. Polyps cause irregular bleeding, very heavy flow, and spotting between periods. Because they create uneven patches of tissue on the lining, they can also cause the lining to shed unevenly, contributing to clot formation.
Adenomyosis occurs when tissue similar to the uterine lining 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 hallmark symptoms are painful periods, heavy or prolonged bleeding, and clots. Adenomyosis is most common in women in their 30s and 40s, particularly those who have had children, though it can occur at any age.
Bleeding Disorders You May Not Know About
Sometimes the problem isn’t with the uterus at all but with your blood’s ability to clot properly. Von Willebrand disease is the most common inherited bleeding disorder, and heavy periods are often its first and most noticeable symptom. Among women with chronically heavy menstrual bleeding, somewhere between 5% and 24% turn out to have von Willebrand disease. Many go undiagnosed for years because heavy periods get dismissed as “just how your body is.”
Von Willebrand disease involves a deficiency or malfunction of a protein that helps platelets stick together. The most common form is mild and causes heavy periods, easy bruising, prolonged bleeding from cuts, and excessive bleeding after dental work or surgery. If you’ve always had heavy, clotty periods, and other women in your family have the same experience, a bleeding disorder is worth investigating. A simple blood test can identify it.
Signs Your Clotting Needs Medical Attention
Occasional small clots during heavier days are normal. What isn’t normal is a pattern of large, frequent clots paired with truly heavy bleeding. Specifically, you should be concerned if you’re passing golf ball-sized clots, passing clots every couple of hours, or soaking through a pad or tampon roughly every hour for two or more hours in a row.
Other warning signs include periods that last longer than seven days, bleeding that regularly disrupts your daily life, or feeling exhausted and lightheaded during or after your period. That last symptom points to iron deficiency anemia, which develops when heavy blood loss over many cycles depletes your body’s iron stores. The telltale signs are persistent fatigue, weakness, dizziness, and shortness of breath. Iron deficiency anemia is common in people with heavy periods and often gets overlooked because the symptoms build gradually.
How the Cause Gets Identified
Figuring out why you’re clotting heavily usually starts with a detailed history of your bleeding patterns: how long your periods last, how often you’re changing pads or tampons, and whether anything has recently changed. From there, a few targeted tests can narrow things down quickly.
Blood work typically includes a complete blood count (to check for anemia), thyroid tests, and sometimes a pregnancy test and screening for sexually transmitted infections, both of which can cause abnormal bleeding. If a bleeding disorder is suspected, specific clotting factor tests are added.
An ultrasound is usually the first imaging step and can reveal fibroids, polyps, and signs of adenomyosis. If more detail is needed, a sonohysterography (where fluid is placed in the uterus during the ultrasound) or a hysteroscopy (where a tiny camera looks inside the uterine cavity directly) can identify smaller polyps or fibroids that a standard ultrasound might miss. In some cases, a small sample of the uterine lining is taken and examined under a microscope to rule out abnormal cell growth.
What Treatment Looks Like
Treatment depends entirely on the underlying cause, which is why identifying that cause matters more than just managing symptoms.
For hormonal imbalances, hormonal birth control is often the first-line approach. Options like the pill, hormonal IUD, or progesterone-only treatments work by thinning the uterine lining, which directly reduces both the volume of bleeding and the clotting. A hormonal IUD in particular can dramatically reduce menstrual flow, and many people find their periods become very light or stop altogether.
Fibroids and polyps can often be removed through minimally invasive procedures, depending on their size and location. For adenomyosis, hormonal treatments can manage symptoms, and in severe cases that don’t respond to other approaches, surgery may be considered. Bleeding disorders like von Willebrand disease are managed with medications that help the blood clot more effectively, and treatment can be tailored to be used only during your period if that’s when symptoms are worst.
If you’ve been living with heavy, clotty periods for a long time, it’s worth knowing that what feels “normal” to you may actually be treatable. Many people don’t realize their bleeding is excessive because they have no frame of reference for what a moderate period looks like. If clots are a regular part of your cycle and you’re tired of working around them, the underlying cause is almost always something that can be identified and addressed.