Passing blood clots during your period is common and usually harmless. Your body naturally sheds the uterine lining each cycle, and when blood pools in the uterus before leaving the body, it can clump together into clots. Small clots, especially during the heaviest days of your period, are a normal part of menstruation. But clots that are consistently the size of a quarter or larger can signal that something else is going on, from hormonal imbalances to structural changes in the uterus.
Why Clots Form During Your Period
Your uterus builds up a blood-rich lining each month in preparation for pregnancy. When pregnancy doesn’t happen, levels of progesterone drop, triggering the lining to break down and shed. As that tissue and blood flow out, your body releases natural anticoagulants to keep things liquid and moving. On heavier days, though, blood can leave the uterus faster than those anticoagulants can work. The result is clots, which are essentially clumps of blood cells, tissue, and proteins that have thickened together.
This is why you tend to see clots on your heaviest flow days (usually days one and two) and less often toward the end of your period. Clots that are small, dark red or maroon, and occasional are generally nothing to worry about.
When Clots Are a Warning Sign
The size and frequency of your clots matter more than whether they appear at all. Passing clots the size of a quarter or larger, especially multiple times a day, points toward abnormally heavy menstrual bleeding. Other signs to pay attention to include soaking through a pad or tampon every one to two hours, bleeding that lasts longer than seven days, and spotting between periods.
Heavy menstrual bleeding isn’t defined by a single number. Clinically, it’s considered excessive when it interferes with your daily life, whether that means skipping activities, doubling up on pads, or feeling physically drained. If large clots are routine for you, it’s worth investigating why.
Uterine Fibroids and Adenomyosis
Fibroids are the most common structural cause of heavy bleeding with clots. These noncancerous growths develop in or on the uterine wall and can range from pea-sized to large enough to distort the shape of your uterus. They cause heavy bleeding in several ways: by increasing the surface area of the uterine lining, by interfering with the uterus’s ability to contract and squeeze blood vessels shut, and by enlarging blood vessels in the uterine wall. When those vessels are wider than normal, the body’s clotting system struggles to seal them off effectively, leading to heavier flow and larger clots.
Adenomyosis is a related condition where tissue that normally lines the uterus grows into the muscular wall instead. It causes the uterus to enlarge, often producing heavy, painful periods with significant clotting. Both fibroids and adenomyosis are extremely common, particularly in women over 30, and are among the first things a doctor will look for on an ultrasound.
Hormonal Imbalances
Your period depends on a careful balance between two hormones. Estrogen builds up the uterine lining during the first half of your cycle. After ovulation, progesterone stabilizes that lining and eventually triggers it to shed. If you don’t ovulate in a given cycle, progesterone never rises, and estrogen continues thickening the lining unopposed. When that overgrown lining finally sheds, there’s simply more tissue and blood to pass, which means heavier bleeding and bigger clots.
Skipped ovulation is surprisingly common. It happens frequently during the first few years of menstruation, during perimenopause, in polycystic ovary syndrome (PCOS), and during periods of significant stress or weight change. A persistently thickened lining, called endometrial hyperplasia, can develop if this pattern continues over time. Left untreated, hyperplasia can in rare cases progress to precancerous changes, which is one reason persistent heavy bleeding deserves medical attention.
Bleeding Disorders
Sometimes the issue isn’t in the uterus at all but in the blood itself. Von Willebrand disease, the most common inherited bleeding disorder, affects up to 1% of the U.S. population and often goes undiagnosed in women for years. Heavy periods may be the most obvious symptom. The CDC notes that women with this condition frequently pass clots larger than grapes or strawberries, soak through a pad every one to two hours, and bleed for longer than seven days.
Clues that a bleeding disorder might be involved include a lifelong history of heavy periods starting from your very first cycle, easy bruising, prolonged bleeding after dental work or minor injuries, and a family history of bleeding problems. Diagnosis involves blood tests that measure how well your clotting proteins function, not just standard blood counts. If your heavy bleeding has been present since adolescence, this is a possibility worth raising with your doctor. Among adolescents with heavy menstrual bleeding, 70% report passing clots and bleeding through clothes and bedding.
Could It Be a Miscarriage?
If your clots look different from what you normally see during a period, pregnancy loss is one possibility. An early miscarriage can feel very similar to a heavy period, especially if you didn’t know you were pregnant. The tissue passed during an early miscarriage often looks like a blood clot mixed with gray-white material, or it may appear as a clear, fluid-filled sac. This is distinct from typical menstrual clots, which are uniformly dark red or maroon.
Other signs that suggest miscarriage rather than a normal period include bleeding that’s significantly heavier than your usual cycle, cramping that feels more intense or different in quality, and a period that arrived later than expected. A pregnancy test can still show positive for days to weeks after a loss, so testing at home can help clarify the situation.
How Doctors Investigate Heavy Clotting
The first step is usually a transvaginal ultrasound, which gives a detailed view of the uterus and can identify fibroids, adenomyosis, polyps, and abnormal thickening of the lining. In one study of women with heavy bleeding who underwent ultrasound, 35% had fibroids, 9% had adenomyosis, and 8% had endometrial polyps. Blood work typically includes a complete blood count and iron levels to check for anemia, plus thyroid function and sometimes clotting studies.
Depending on results, further evaluation might include a saline ultrasound (which uses fluid to get a clearer look inside the uterus) or a biopsy of the uterine lining, particularly for women over 40 or those with risk factors for endometrial hyperplasia.
The Anemia Connection
Losing large clots regularly means losing iron. Every milliliter of blood that leaves your body takes about 0.4 to 0.5 milligrams of iron with it. Over months or years of heavy periods, this can quietly deplete your iron stores and lead to iron deficiency anemia. Symptoms include fatigue, shortness of breath during normal activities, dizziness, pale skin, and difficulty concentrating.
Anemia shows up on blood work as a hemoglobin level below 12 g/dL, but your iron stores (measured by a protein called ferritin) can be dangerously low even when your hemoglobin looks normal. Ferritin below 30 is considered iron deficient. If you’ve been living with heavy periods for years, you may have normalized symptoms like exhaustion and brain fog that are actually treatable iron deficiency.
What Treatment Looks Like
Treatment depends entirely on the cause. Hormonal options, including birth control pills, hormonal IUDs, and other progesterone-based therapies, work by thinning the uterine lining so there’s less to shed each month. Birth control pills also raise levels of clotting proteins in the blood, which can reduce bleeding even in women with von Willebrand disease.
For fibroids or polyps, procedures range from minimally invasive removal of the growth to, in severe cases, surgery on the uterus itself. Many women with fibroids see significant improvement from a hormonal IUD alone, without needing surgery. For women whose heavy bleeding stems from anovulation, restoring regular ovulation through hormonal treatment or addressing the underlying cause (like thyroid dysfunction or PCOS) often resolves the clotting.
If your clots are small and your periods don’t disrupt your life, no treatment is necessary. But if you’re soaking through protection every hour or two, passing quarter-sized clots regularly, or feeling the cumulative effects of blood loss, these are problems with real solutions.