Passing small blood clots during your period is normal. Your body naturally produces substances that break down clot-forming proteins inside the uterus, keeping menstrual blood fluid as it exits. But when your flow is heavy enough to outpace that process, clots form. Small clots, roughly the size of a raisin, are common on heavier days. Clots bigger than a grape, or especially quarter-sized and larger, signal that something may be increasing your flow beyond what your body can manage.
How Your Body Prevents Clots (and Why It Fails)
During your period, the uterine lining breaks down and bleeds. To keep that blood from clotting inside the uterus, the endometrium releases enzymes that dissolve clot-forming proteins as fast as they appear. Think of it as a built-in anticlotting system designed to keep menstrual blood liquid.
This system works well at normal flow volumes. When bleeding is heavier or faster than usual, though, the enzymes can’t keep up. Blood pools in the uterus, clotting proteins accumulate, and the result is visible clots that pass when you stand up, change positions, or use the bathroom. The heavier the flow, the larger and more frequent the clots.
Hormonal Imbalances That Thicken the Lining
The most common reason for a heavier-than-normal period is a hormone imbalance between estrogen and progesterone. Estrogen thickens the uterine lining during the first half of your cycle, building up tissue and blood supply in preparation for pregnancy. Progesterone, released after ovulation, stabilizes that lining and triggers it to shed in an orderly way.
When ovulation doesn’t happen (which is surprisingly common during stress, after stopping birth control, during perimenopause, or with conditions like PCOS), progesterone never kicks in. Estrogen keeps building the lining unopposed, sometimes for weeks. By the time your period finally arrives, there’s significantly more tissue and blood to shed, which means a heavier flow and bigger clots. In some cases, prolonged estrogen exposure without progesterone leads to a condition called endometrial hyperplasia, where the lining grows abnormally thick. Heavier or longer-than-usual bleeding is the hallmark symptom.
Fibroids and Adenomyosis
Structural changes in the uterus itself are another major cause. Uterine fibroids are noncancerous growths in or on the uterine wall. When they grow near the inner lining, they increase the surface area that bleeds during your period and can distort the uterus in ways that make it harder for it to contract and slow bleeding. The result is a noticeably heavier flow with larger clots.
Adenomyosis is a related but different condition where the tissue that normally lines the uterus grows into the muscular wall. During your period, that embedded tissue also thickens, breaks down, and bleeds, just like the regular lining. This causes the uterus to enlarge over time and produces painful periods with heavy bleeding and clots. Adenomyosis is most common in women in their 30s and 40s, especially after childbirth, though it can happen earlier.
Thyroid Problems and Bleeding Disorders
Your thyroid gland, which controls metabolism, also influences your menstrual cycle through its effects on reproductive hormones. An underactive thyroid can alter levels of key hormones that regulate ovulation, including prolactin. When ovulation is disrupted, the same estrogen-dominance pattern described above takes hold, leading to heavier periods and clotting.
Bleeding disorders are an underrecognized cause. Von Willebrand disease, the most common inherited bleeding disorder, affects the blood’s ability to clot properly throughout the body. Among women with chronic heavy menstrual bleeding, somewhere between 5% and 24% turn out to have von Willebrand disease. Many go undiagnosed for years because heavy periods are often dismissed as “just how it is.” If you’ve had heavy periods since your very first cycle, bruise easily, or have prolonged bleeding after dental work or minor cuts, a bleeding disorder is worth investigating.
Could It Be a Pregnancy Loss?
An unusually heavy period with large clots can sometimes be an early miscarriage, particularly if your period arrived late or felt different from your usual pattern. Early pregnancy loss can look very similar to a heavy period, but there are some distinguishing features: passage of grayish or pinkish tissue (not just dark red clots), a gush of clear or pink fluid, cramping that feels more intense than your normal period pain, or the sudden disappearance of pregnancy symptoms like breast tenderness or nausea. If there’s any chance you could have been pregnant, getting evaluated is important. Tissue passage can occasionally occur with an ectopic pregnancy, which requires prompt treatment.
Signs Your Clots Need Medical Attention
The American College of Obstetricians and Gynecologists defines heavy menstrual bleeding by specific benchmarks. Any of the following qualifies:
- Clots the size of a quarter or larger
- Soaking through a tampon or pad every hour for several consecutive hours
- Needing to double up on pads
- Having to change pads or tampons overnight
- Bleeding that lasts more than 7 days
Even one of these is enough to warrant a workup. Heavy menstrual bleeding has identifiable causes in the vast majority of cases, and most of them are treatable.
How Heavy Periods and Clots Are Treated
Treatment depends on the underlying cause, but several options can reduce flow volume and clot size regardless of the reason.
Hormonal birth control (pills, hormonal IUDs, patches) works by thinning the uterine lining so there’s less tissue to shed each cycle. For many women this dramatically reduces both flow and clotting. A hormonal IUD in particular can reduce menstrual bleeding by more than 80% in some cases, and some women stop getting periods altogether.
For women who can’t or prefer not to use hormonal methods, tranexamic acid is a non-hormonal medication taken only during your period. It works by helping the body’s natural clotting system work more efficiently. In a randomized controlled trial, women taking tranexamic acid saw their menstrual blood loss drop by about 40%, compared to only 8% in the placebo group. Over half of treatment cycles saw a reduction of at least 50 mL, a clinically meaningful decrease.
When fibroids or adenomyosis are the cause, treatment ranges from medication to manage symptoms to procedures that remove or shrink the growths. The right approach depends on the size and location of fibroids, symptom severity, and whether you plan to have children in the future. For thyroid-related heavy bleeding, treating the thyroid condition itself often normalizes periods without additional intervention. And for bleeding disorders like von Willebrand disease, targeted therapies can improve clotting function and reduce menstrual blood loss significantly.