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Illustration by Katrin Friedmann


Blood clots during your period: what are they?

Top things to know:

  • Menstrual blood clots are normal on the heaviest days of your period

  • The exact cause of menstrual blood clots is unclear

  • If your menstrual bleeding is heavy and your clots are larger than about the size of a quarter, it's a good idea to talk with your healthcare provider

You typically know your period has begun when you see a little, or a lot, of blood in your underwear. At some points during menstruation you may notice clumps or chunks of blood on your collection methods or in your underwear. Well, what are they?

The shedding of your uterine lining (your endometrium) will typically have a varying flow volume that gradually, or suddenly, increases. Menstrual bleeding also varies in duration, color (from old or new blood), and texture. Period blood texture varies as a result of shedding endometrial tissue that can come out as blood clots.

Period blood clots are a common feature of menstrual blood—they can appear like clumps, chunks, and/or a jelly-like consistency and can vary in size and number.

An average period sheds about 2 to 3 tablespoons (35 to 40 milliliters) of blood and tissue. Noticing clots or clumps, especially on the heaviest days, is normal.

Menstrual blood clots are distinct from venous blood clots (the kind discussed in relation to hormonal birth control). Blood clots are the result of coagulation—the process of making fluid blood to a semi-solid or solid state. Blood clots that occur from circulating blood are necessary to stop bleeding from injury, but they can also form unnecessarily and cause medical problems.

It's generally accepted that your body's anticoagulants are behind period blood clots (1). According to The Center for Young Women's Health at Boston Children's Hospital, your body releases these substances to prevent menstrual blood from clotting as it's shedding, but when flow volume is heavy, there's not always enough time for anticoagulants to come into full effect, which results in blood clot formation.

There is conflicting evidence on where period clots form — either in the vagina or in the uterine cavity (2, 3). Blood coagulation is generally understood to occur from the interaction of two proteins: thrombin and fibrinogen. However, some research claims menstrual blood contains neither thrombin nor fibrinogen (2, 3). One early study suggested the fluidity of menstrual blood is a result of a lytic agent that dissolves previously clotted blood (2). A lytic agent is a substance that dissolves something. So this could mean period blood clots are formations that didn't get thoroughly liquefied by a lytic agent.

Other studies had a different take on the role of fibrinogen and period blood clots. Plasminogen activators are a group of enzymes that cause fibrinolysis—the dissolving of blood clots. Research found people who suffer from HMB have more enzymes in the lining of the uterus that dissolve blood clots (4, 5).

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Other studies state menstrual blood clots are red cell aggregations to mucus-related substances, and claim mucosa found in premenstrual phase is meant to liquify menstrual blood (2, 3).

Although there is inconsistency on what exactly causes period clots, they're a common and a normal characteristic of period blood on heavy flow volume days.

If you have multiple blood clots larger than the size of a quarter that can be an indication of heavy menstrual bleeding (HMB). Large and greyish clots may be from an undetected miscarriage.

If you take oral anticoagulants—medication to prevent blood clots—you may experience heavy and prolonged menstrual bleeding (6, 7). According to UpToDate, up to 15 to 24 percent of women with menorrhagia may have some type of bleeding condition, such as von Willebrand disease, immune thrombocytopenia, or platelet function defect.

Bleeding from other parts of your body occurs from injury or illness, so menstrual bleeding can appear differently, as it's an unrelated process. Don't always expect period blood to be a consistent and pure red fluid.

Know your body. Download Clue today and track aspects of your period.

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