Blood clots during your period are gel-like clumps made of blood, tissue from the uterine lining, and mucus. They’re a normal part of menstruation for most people, especially on heavier flow days. Small clots, typically dark red or almost black, pass without issue. Clots become a concern when they’re larger than a quarter (about 2.5 cm across) or show up alongside other signs of unusually heavy bleeding.
What Menstrual Clots Are Made Of
Each month, your uterus builds up a thick, blood-rich lining in preparation for a possible pregnancy. When pregnancy doesn’t happen, that lining sheds. What comes out is a mix of blood, blood byproducts, mucus, and pieces of endometrial tissue. A menstrual clot forms when some of this material coagulates before or as it leaves your body, creating those jelly-like blobs you might notice on a pad, tampon, or in the toilet.
Your body actually has a built-in system to keep menstrual blood flowing smoothly. The uterus releases enzymes that break down clots (part of what’s called the plasminogen activator system), essentially thinning the blood so it can exit easily. On lighter days, this system keeps up. On heavier days, the volume of blood can outpace the enzymes’ ability to break everything down, so clots form. This is why you’re most likely to see clots on day one or two of your period, when flow is heaviest.
What Color and Size Tell You
Clot color depends mainly on how long the blood has been sitting in your uterus before it exits. Bright red clots are fresh and typically appear during your heaviest flow. Dark red or brown clots have had more time to react with oxygen, a process called oxidation. Black clots, which can look alarming, are simply old blood that sat in the uterus even longer, often appearing at the very beginning or tail end of your period. None of these colors on their own signal a problem.
Size matters more than color. Small clots, from a few millimeters up to the size of a dime, are common and not worrisome. The CDC uses a quarter (roughly the size of a 10-pence coin) as the benchmark: if you’re regularly passing clots that size or larger, that’s considered a sign of heavy menstrual bleeding, also called menorrhagia, and it’s worth investigating.
Why Some People Get More Clots
Occasional clots are normal, but consistently heavy clotting often has an underlying cause. Several conditions can shift your period from manageable to overwhelming.
Fibroids
Uterine fibroids are noncancerous growths in the wall of the uterus. They increase the surface area of the uterine lining, which means there’s simply more tissue to shed each cycle. Fibroids also disrupt the blood vessels and clotting mechanisms within the uterus, leading to heavier bleeding and larger clots. They’re extremely common, affecting up to 70-80% of women by age 50, though not all fibroids cause symptoms.
Adenomyosis
In adenomyosis, tissue similar to the uterine lining grows 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 result is painful periods with prolonged, heavy bleeding and significant clotting. It’s often diagnosed in people in their 30s and 40s, though it can occur earlier.
Hormonal Imbalances
Your uterine lining grows in response to estrogen and is stabilized by progesterone. When these hormones are out of balance, the lining can build up thicker than normal, producing a heavier, clottier period when it finally sheds. Conditions like polycystic ovary syndrome (PCOS), thyroid disorders, and perimenopause can all disrupt this balance. Hormonal shifts are also why teenagers and people approaching menopause tend to experience more clotting.
Copper IUDs
Copper intrauterine devices are a known trigger for heavier periods, particularly in the first several months after insertion. Research has found greater clot-dissolving enzyme activity in the uterine lining of people with copper IUDs, which sounds counterintuitive but reflects the body’s response to more bleeding overall. Heavier flow means more opportunities for clots to form when that enzyme system can’t keep pace.
When Clotting Signals a Problem
A few small clots on your heaviest day are routine. The pattern to watch for is a combination of signs that together point to heavy menstrual bleeding:
- Clots the size of a quarter or larger appearing regularly, not just once
- Soaking through a pad or tampon every hour for several consecutive hours
- Periods lasting longer than seven days
- Needing to change protection overnight or doubling up on products
Any one of these on its own is worth noting, but if you’re checking multiple boxes, something beyond normal variation is likely going on.
How Heavy Clotting Leads to Anemia
The biggest downstream risk of persistently heavy periods with clotting is iron deficiency anemia. Every time you lose blood, you lose iron. Your body uses iron to make hemoglobin, the protein in red blood cells that carries oxygen to your tissues. When monthly blood loss consistently exceeds what your body can replenish, iron stores drop, hemoglobin falls, and oxygen delivery suffers.
The symptoms creep up gradually, which is why many people don’t connect them to their period. Persistent fatigue that doesn’t improve with sleep is the hallmark. Shortness of breath during activities that used to feel easy, frequent headaches, dizziness, and feeling cold all the time are other common signs. If you’ve been told your heavy periods are “just how it is” but you’re also exhausted and pale, anemia from blood loss is a likely explanation.
How Heavy Clotting Is Managed
Treatment depends on the cause, but the goal is almost always the same: reduce the amount of bleeding so fewer clots form in the first place.
Hormonal options are the most common starting point. Birth control pills, hormonal IUDs, and other hormonal methods thin the uterine lining over time, which directly reduces the volume of tissue and blood shed each cycle. Many people see a dramatic reduction in both flow and clotting within a few months.
For people who can’t or prefer not to use hormonal methods, there’s a non-hormonal prescription medication that works by preventing clots from breaking down once they’ve formed in the uterine lining. This stabilizes the bleeding rather than thinning the lining. It’s taken only during your period, typically for up to five days per cycle, and can significantly reduce both volume and clot size.
When a structural cause like fibroids or adenomyosis is driving the problem, treatment may involve procedures to remove fibroids, reduce the uterine lining, or in more severe cases, address the uterus itself. The approach depends on how much the bleeding affects your quality of life, whether you want to preserve fertility, and how your body responds to less invasive options first.
Iron supplementation is often recommended alongside any of these treatments if blood tests confirm low iron stores. Rebuilding iron levels can take several months, but most people notice their energy improving within a few weeks of starting supplements.