Period blood clots are pieces of thickened blood and uterine tissue that your body sheds during menstruation. Small clots, roughly the size of a raisin or smaller, are a normal part of heavier flow days and don’t typically signal a problem. Clots larger than about 2.5 cm (roughly the size of a US quarter) or clots that show up frequently throughout your period can point to an underlying cause worth investigating.
How Period Clots Form
During your period, your uterus sheds its inner lining, which is a mix of blood, tissue, and mucus. Your body normally releases anticoagulants, natural blood-thinning substances, to keep this material flowing smoothly out of the uterus. On heavier flow days, blood can pool in the uterus faster than those anticoagulants can work, so the blood has time to thicken and form clots before it exits your body.
This is why clots tend to appear on your heaviest days, often the first two or three days of your period, and why they’re less common toward the end when flow is lighter. The color ranges from bright red to dark maroon or even blackish, depending on how long the blood sat in the uterus before passing.
A Thicker Uterine Lining Means More Clots
The thickness of your uterine lining plays a direct role in how much material your body has to shed. Estrogen is the hormone responsible for building up that lining each cycle, thickening it in preparation for a potential pregnancy. Progesterone, released after ovulation, stabilizes the lining and triggers it to shed in an orderly way when pregnancy doesn’t occur.
When estrogen levels run high relative to progesterone, a pattern sometimes called estrogen dominance, the lining can grow unusually thick. If you skip ovulation in a given cycle (which happens more often than you might think, especially during perimenopause, after stopping birth control, or with conditions like PCOS), progesterone isn’t produced on schedule. Without that signal, the endometrium keeps growing in response to estrogen. When it finally sheds, the volume of blood and tissue overwhelms your body’s anticoagulants, producing larger and more frequent clots.
This kind of hormonal imbalance is one of the most common explanations for clotty periods, particularly if your cycle length has also become irregular.
Fibroids and Structural Changes
Uterine fibroids are noncancerous growths in or on the uterus, and they’re remarkably common. By age 50, the majority of women have had at least one. Fibroids contribute to clotting in a few ways. They increase the surface area of the uterine lining, meaning there’s simply more tissue shedding each cycle. They also affect blood vessels and clotting mechanisms within the uterus, disrupting the normal regulation of blood flow during menstruation. Fibroids that grow into the uterine cavity (submucosal fibroids) tend to cause the heaviest bleeding and the largest clots.
Adenomyosis, a related condition where uterine lining tissue grows into the muscular wall of the uterus, produces similar effects. It makes the uterus larger and spongier, leading to heavier, more painful periods with noticeable clotting. Adenomyosis is often underdiagnosed because its symptoms overlap so much with fibroids and general heavy periods.
Endometriosis and Heavy Bleeding
Endometriosis occurs when tissue similar to the uterine lining grows outside the uterus, on the ovaries, fallopian tubes, or other pelvic structures. This tissue responds to hormonal signals the same way the lining inside the uterus does: it thickens, breaks down, and bleeds with each cycle. But unlike normal menstrual blood, it has nowhere to go. The trapped blood causes inflammation and scarring internally.
Within the uterus itself, endometriosis can also drive heavier menstrual bleeding. The Endometriosis Foundation of America notes that this often includes blood clots. If your clotty periods come with severe cramping, pain during sex, or pain between periods, endometriosis is worth discussing with a gynecologist. Diagnosis usually requires imaging or, in some cases, a minor surgical procedure.
Bleeding Disorders and Nutritional Gaps
Your body relies on a cascade of clotting factors to control bleeding throughout the body, including inside the uterus. When any of those factors are impaired, periods can become heavier and clottier. More than half of women with unexplained heavy periods show laboratory signs of impaired clotting, and sometimes heavy periods are the first or only symptom of a bleeding disorder.
Vitamin K is essential for producing several of those clotting factors. A deficiency leads to incomplete activation of these proteins, resulting in defective clotting and an increased risk of prolonged bleeding. While severe vitamin K deficiency is uncommon in people eating a varied diet, milder insufficiencies from poor dietary intake or conditions that impair nutrient absorption can contribute to heavier menstrual bleeding. In documented cases, menstrual bleeding improved with vitamin K replacement. Green leafy vegetables, broccoli, and fermented foods are the richest dietary sources.
Iron deficiency works in the opposite direction: it doesn’t cause clots, but heavy, clotty periods are one of the leading causes of iron-deficiency anemia in women. Symptoms include fatigue, shortness of breath, dizziness, and feeling cold. Severe anemia from ongoing blood loss increases the risk of heart problems over time.
What Size Clot Is Considered Abnormal
The general benchmark used by medical organizations is the size of a coin, roughly 2.5 cm or about the diameter of a US quarter. Occasional clots smaller than that on your heaviest day are considered within the normal range. Clots that size or larger, or smaller clots that appear consistently throughout your period, suggest your bleeding volume is high enough to warrant evaluation.
Other signs that your period is heavier than normal include soaking through a pad or tampon every hour for several consecutive hours, needing to double up on protection, bleeding that lasts longer than seven days, or feeling exhausted and lightheaded during your period. These symptoms together paint a clearer picture than clot size alone.
How Heavy, Clotty Periods Are Managed
Treatment depends entirely on the underlying cause. If hormonal imbalance is driving a thicker lining, hormonal options like birth control pills, hormonal IUDs, or progesterone therapy can thin the endometrium and reduce both flow and clotting. Many people notice a significant difference within two to three cycles.
For those who prefer non-hormonal options, there are medications that work by helping your body’s own clotting system perform better during your period. These are taken only during the days of active bleeding, typically for up to five days per cycle, and can meaningfully reduce both clot size and overall blood loss.
When fibroids or adenomyosis are the cause, the approach depends on the size, location, and number of growths, as well as whether you want to preserve fertility. Options range from medications that shrink fibroids to procedures that remove or destroy them. For adenomyosis, hormonal treatments are usually tried first.
If a nutritional deficiency is contributing, correcting that deficiency (through diet or supplementation) can improve clotting function over time. Iron supplementation is commonly recommended alongside treatment for heavy periods to rebuild depleted stores, and it can take several months before energy levels fully recover.