Passing blood clots during your period is your body’s way of managing a heavier-than-usual flow, and it happens when your natural clot-prevention system can’t keep up with the volume of blood leaving your uterus. Small clots are common and typically harmless. But if you’re consistently passing clots the size of a quarter (about one inch across) or larger, something is likely driving your flow beyond what’s considered normal.
How Clots Form During Your Period
Your uterus has a built-in mechanism to keep menstrual blood liquid as it leaves your body. Cells lining the blood vessels in your uterus release a protein called tissue plasminogen activator, which breaks down clots before they fully form. Think of it as a natural blood thinner working right at the source.
When your period is light to moderate, this system handles everything smoothly, and your menstrual blood flows out without clumping. But when bleeding is heavy or fast, the system gets overwhelmed. There simply isn’t enough of that clot-dissolving protein to process all the blood at once. The blood pools in your uterus, begins to coagulate, and forms the dark, jelly-like clots you see on your pad or in the toilet. The clots are a mix of blood cells, uterine lining tissue, and proteins involved in clotting. Their deep red or dark purple color comes from sitting in the uterus before being expelled.
A Thick Uterine Lining Is the Most Common Cause
The amount of tissue your uterus builds up each cycle directly determines how much material needs to come out during your period. Estrogen is the hormone responsible for thickening that lining in the first half of your cycle. After ovulation, progesterone rises to stabilize the lining and eventually trigger its shedding. When this balance works properly, the lining stays a manageable thickness.
Problems start when estrogen runs high relative to progesterone. If you don’t ovulate in a given cycle (which can happen due to stress, polycystic ovary syndrome, perimenopause, or other reasons), progesterone never rises to counterbalance estrogen. The lining keeps growing thicker than it should. When it finally sheds, the volume of blood and tissue is significantly greater, producing more clots. This overgrowth is called endometrial hyperplasia, and it’s one of the most straightforward explanations for cycles that suddenly seem heavier and clottier than usual.
This hormonal pattern is especially common during two life stages: the first few years of menstruation and the years leading up to menopause. Both are times when ovulation is irregular, making estrogen-dominant cycles more frequent.
Adenomyosis and Fibroids
If your heavy, clot-filled periods have been getting progressively worse over months or years, a structural issue with the uterus itself may be involved. Adenomyosis is a condition where the tissue that normally lines the inside of the uterus grows into the muscular wall. That displaced tissue still responds to your hormones each month: it thickens, breaks down, and bleeds, but now it’s doing so inside the muscle. This causes the uterus to enlarge and produce significantly heavier periods with more clots. You may also notice deep, aching cramps and a feeling of pressure or tenderness in your lower abdomen.
Uterine fibroids, which are noncancerous growths in or on the uterus, can have a similar effect. Fibroids that grow into the uterine cavity distort the lining and increase the surface area that bleeds each cycle. Both conditions are common, particularly in your 30s and 40s, and both are identifiable through imaging like an ultrasound.
Copper IUDs and Other Triggers
If your clotting started after getting a copper IUD, the device is a likely culprit. Copper IUDs are well known for increasing menstrual bleeding and cramping, and passing blood clots is a recognized side effect that can persist for several months to a full year after insertion. Hormonal IUDs tend to have the opposite effect, often making periods lighter or stopping them altogether. If your copper IUD is causing problems that don’t improve, switching to a hormonal option is worth discussing.
Other factors that can increase clotting include bleeding disorders (some of which run in families), certain medications that thin the blood, and, less commonly, growths like endometrial polyps. If anyone in your family has a known clotting disorder, that’s important information to share with a healthcare provider.
How to Tell If Your Clotting Is Too Heavy
Some clotting is normal, so the question isn’t whether you pass clots at all but how big they are and what else is happening alongside them. The CDC uses the size of a U.S. quarter as the threshold: clots that size or larger signal heavy menstrual bleeding that warrants evaluation. Other signs that your bleeding has crossed from heavy-but-normal into territory that needs attention include:
- Soaking through a pad or tampon every one to two hours for several consecutive hours
- Periods lasting longer than seven days
- Needing to double up on pads or change them overnight
- Feeling dizzy, lightheaded, or unusually fatigued
That last point matters more than people realize. Persistently heavy periods are one of the most common causes of iron deficiency anemia in people who menstruate. Every clot you pass contains red blood cells packed with iron. Over months of heavy bleeding, your iron stores drop, and your body can’t produce enough hemoglobin to carry oxygen efficiently. The result is fatigue that doesn’t improve with sleep, shortness of breath during normal activities, and sometimes pale skin or brittle nails. Many people chalk this up to being busy or stressed when their periods have been quietly draining their iron for months.
Treatment Options That Reduce Clotting
Treatment depends on what’s driving the heavy bleeding, but several approaches specifically target clot formation and flow volume. Hormonal birth control (pills, hormonal IUDs, or patches) works by thinning the uterine lining so there’s less tissue to shed each cycle. This is often the first option offered, and it addresses the estrogen-progesterone imbalance that causes excessive lining buildup.
For people who prefer a non-hormonal option, there’s a medication that works by stabilizing clots rather than preventing them. It’s taken as a tablet up to three times daily during the heaviest days of your period, for no more than five consecutive days per cycle. It won’t stop your period but can meaningfully reduce the volume of bleeding and the size and number of clots.
When a structural issue like fibroids or adenomyosis is the underlying cause, treatment may involve procedures to remove the growths or, in more severe cases, address the uterine tissue directly. An ultrasound is typically the first step in identifying whether something structural is going on. If your periods have changed noticeably, getting that imaging can save you months of guessing.
Iron supplementation is important if you’ve already developed anemia from ongoing heavy bleeding. Replenishing iron stores takes time, often several months of consistent supplementation, so starting sooner rather than later makes a real difference in how quickly your energy returns.