Why Are There Blood Clots in My Period?

Most of the time, they're a normal part of menstruation

Woman sitting on toilet bowl with hand on chin, mid section
Hitoshi Nishimura/Getty Images

For most women, clots are part and parcel of period blood. In fact, it's normal, in fact necessary, for blood to clot. Clotting is the body's mechanism for stopping bleeding. The scab that develops on a cut is in part clotted blood that hardens to create a sort of natural bandage on the wound. It stands to reason, then, that it's more problematic if blood doesn't clot, which can be a symptom of an underlying health problem or a side effect of a medication that alters the production or function of the body’s clotting factors.

Clots in menstrual blood aren't exactly scabs, but they're rarely something to worry about either. Here's how normal clumps in your period to form, what they're made up of, and how to know when they're a sign something could be wrong.

Is It a Clot – or Not?

Blood clots form when hormones trigger the body to start shedding the uterine lining, exposing small blood vessels and causing them to bleed. While this blood is waiting to pass through the cervix and vagina during menstruation, clots can form. These are usually quite small and dark red to almost black in color, a result of sitting in the uterus for awhile. 

Mixed into menstrual blood are also tiny bits of tissue from the small vessels in the lining of the uterus, the endometrium. Sometimes what appears to be a blood clot is actually a clump of endometrial cells, or a mixture of clump and clot. 

During a heavy menstrual flow, blood clots tend to be bigger because there is a larger amount of blood sitting in the uterus that needs to clot.

They usually are much brighter red too. This is because blood is filling the cavity of the uterus more quickly and moving out more quickly—so quickly it doesn't have time to darken. This explains in part why, if you have a heavy flow, you're more likely to have painful cramping. In order to pass larger blood clots, the cervix has to dilate a bit, causing pain that can be quite intense.

Why Size Matters

How big is big when it comes to menstrual blood clots? If you have very heavy bleeding and are passing clots the size of a grape, see your doctor. Depending on your age and your medical history, any number of things could be pumping up the volume of your menstrual flow and causing bigger-than-average clots to form, including uterine fibroids, endometrial polyps, or adenomyosis. To test for these conditions, your caregiver may order you to have a blood test to check for a bleeding disorder, an ultrasound, and/or an endometrial biopsy.

It isn't normal for a young girl who's just begun menstruation to have a heavy flow and large blood clots either. If you have just started having your period and your flow is very heavy with larger blood clots this isn't normal. Tell a parent or the school nurse that you need to see a doctor.

View Article Sources
  • Barbara S. Apgar, M.D., M.S., Amanda H. Kaufman, M.D., Uche George-nwogu, M.D., and Anne Kittendorf, M.D. University of Michigan Medical Center.  Treatment of Menorrhagia. American Fam Physician. 2007 Jun 15;75(12):1813-1819.