Blood clots during your period are a normal part of menstruation, and most people who menstruate pass them regularly, especially on their heaviest days. Clots form when blood pools in your uterus or vagina faster than your body’s natural clot-dissolving system can keep up. Small clots the size of a raisin or pea are nothing to worry about. Clots larger than a grape, or a sudden increase in clotting compared to your usual pattern, can signal something worth investigating.
How Period Clots Actually Form
Your uterine lining spends weeks building up in preparation for a potential pregnancy. When that pregnancy doesn’t happen, progesterone levels drop sharply, and this hormonal withdrawal triggers menstruation. The drop in progesterone causes the specialized blood vessels in the uterine lining (called spiral arterioles) to constrict and then break down, releasing blood and tissue into the uterine cavity.
Your body has a built-in system to keep this blood flowing smoothly. Enzymes called plasminogen activators break down fibrin, the protein that forms the scaffolding of blood clots. Think of it as a natural blood thinner that works specifically inside your uterus. On light-flow days, this system easily dissolves clots before they leave your body. On heavy-flow days, blood collects in the uterus faster than those enzymes can work, so clots pass intact. That’s why you tend to see the most clots on days one and two of your period, or first thing in the morning after blood has pooled overnight.
Why Some Periods Produce More Clots Than Others
The thickness of your uterine lining directly determines how much blood and tissue your body sheds. A thicker lining means more material, more blood, and more clots. Several things influence how thick that lining gets.
Estrogen is the hormone responsible for building the lining each cycle. If your body produces more estrogen relative to progesterone, the lining grows thicker than usual. This imbalance is common during perimenopause, after a skipped ovulation cycle, or in people with polycystic ovary syndrome. When ovulation doesn’t occur in a given cycle, progesterone is never produced, so the lining keeps growing in response to estrogen without the usual signal to stop. The result is a heavier, clottier period when the lining finally sheds.
Stress, weight changes, new medications, and changes to hormonal birth control can all shift the balance enough to produce a noticeably different period from one month to the next. A single unusually clotty period after a stressful month or a cycle where you ovulated late is common and not necessarily a sign of a problem.
Structural Causes: Fibroids and Adenomyosis
If heavy, clotty periods are your norm rather than an occasional occurrence, a structural issue in the uterus is one of the most common explanations. Uterine fibroids are noncancerous growths in or on the uterine wall. They can distort the uterine cavity, increase the surface area of the lining, and interfere with the uterus’s ability to contract and squeeze blood vessels shut after shedding. All of this adds up to heavier flow and larger clots.
Adenomyosis is a related condition where the tissue that normally lines the uterus grows into the muscular wall itself. That embedded tissue still responds to your monthly hormonal cycle: it thickens, breaks down, and bleeds, but it’s trapped inside the muscle. This causes the uterus to enlarge and produce heavier, longer, more painful periods. Both fibroids and adenomyosis are extremely common, particularly in people over 30, and are diagnosed with an ultrasound or MRI.
Bleeding Disorders Are More Common Than You’d Think
Among people with chronically heavy periods, somewhere between 5% and 24% have an underlying bleeding disorder that has never been diagnosed. The most common one is von Willebrand disease, an inherited condition that affects how well your blood clots throughout your body, not just during your period. It affects roughly 1 in 100 people, but many don’t know they have it because heavy periods are so often dismissed as “just how your body works.”
Clues that a bleeding disorder might be involved include heavy periods starting from your very first cycle as a teenager, a history of easy bruising, prolonged bleeding after dental work or minor cuts, or a family member who also bleeds heavily. A blood test can check for von Willebrand disease and other clotting factor deficiencies.
How to Tell Normal Clots From a Problem
The size and frequency of your clots matter more than whether they exist at all. Clots smaller than a grape that appear mostly on your heaviest days fall within the normal range. Clots that are consistently larger than a grape, especially if they appear throughout your period rather than just on heavy days, are worth mentioning to a healthcare provider.
The total volume of your flow is another useful measure. A typical period involves losing about 30 to 40 mL of blood across the entire cycle. Heavy menstrual bleeding is defined as losing more than 80 mL per cycle, which works out to soaking through roughly 16 regular-sized pads or tampons over the full period. A more practical threshold: if you’re regularly soaking through nine to twelve products per period, your flow qualifies as heavy.
Some situations call for more urgent attention. If your flow is heavy enough that you feel faint or dizzy when you stand up, that warrants a same-day medical appointment. Soaking through a pad or tampon every hour for several consecutive hours is another sign that your bleeding has crossed into territory that needs evaluation.
What Helps Reduce Heavy Flow and Clotting
Treatment depends on what’s driving the heavy bleeding, but several options can significantly reduce flow regardless of the cause.
Anti-inflammatory medications taken from the start of your period through the end of bleeding can reduce flow substantially. In clinical trials, one common prescription anti-inflammatory reduced reports of heavy bleeding by 56% compared to a placebo. These medications work by shifting the balance of prostaglandins in the uterus, promoting better constriction of the blood vessels that feed the lining.
Medications that support your body’s clot-stabilizing system take a different approach. Instead of preventing clots from forming, they prevent clots from breaking down too quickly. In people with heavy periods, this system is often overactive, dissolving clots faster than normal on the second day of bleeding. Blocking that overactivity can reduce blood loss by a meaningful amount, sometimes more effectively than anti-inflammatories alone.
Hormonal options, including certain IUDs, birth control pills, and other hormonal methods, work by thinning the uterine lining so there’s simply less tissue to shed each month. For fibroids or adenomyosis that don’t respond well to medication, procedures ranging from minimally invasive to surgical can address the underlying structural problem. The right approach depends on your specific situation, whether you want to preserve fertility, and how much the bleeding is affecting your daily life.
Why Your Clotting Pattern Can Change Over Time
Your periods at 35 will not look like your periods at 20, and that shift often includes more clotting. As you move through your reproductive years, hormonal fluctuations become less predictable. Cycles where ovulation is delayed or skipped become more frequent in the years leading up to menopause, and each of those cycles tends to produce a thicker lining and a heavier period when it finally arrives. Fibroids also grow more common with age and can appear or enlarge without any obvious symptoms other than changing periods.
A gradual increase in clotting over months or years is a different situation from a sudden, dramatic change. Both are worth tracking, but a sudden shift, especially if accompanied by pain, periods lasting longer than eight days, or bleeding between periods, provides more useful diagnostic information. Keeping a simple record of how many products you use per period and noting any clots larger than a grape gives your provider concrete data to work with rather than a vague sense that things have changed.