Period Blood Clots: Why They Happen And What They Mean

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It is understandable why you may be concerned if you see blood clots in your menstrual flow. For most women, this is just a normal part of menstruation and nothing to worry about.

Why Do Period Blood Clots Happen?

In order to understand why you might have blood clots in your period, you need to understand what your menstrual flow is and how it happens. Your menstrual flow is a mixture of the:

  • cells from the shedding of the lining of your uterus or endometrium
  • blood that comes from the small vessels in the wall of your uterus that are uncovered when the lining of your uterus is shed

Sometimes the cells of the shed endometrium clump together and appear to be a clot of blood when really it is just a clump of tissue or endometrium.

The cyclic hormone changes in your body trigger the shedding of the lining of your uterus or the endometrium. During this process, the small blood vessels under the surface of the endometrium are exposed and actively bleed. This continues until clotting factors and other changes in the wall of the uterus stop them from bleeding.

It is very normal for your blood to clot. In fact, that is exactly how your body stops itself from bleeding. And your blood contains certain factors that cause this clotting to happen. So, unless you have an underlying medical problem or you are taking a medication that affects the production or function of your body’s clotting factors or platelets your blood will clot.

During your period the cells shed from the lining of the walls of your uterus and the blood from the exposed small vessels collect inside your uterine cavity before it passes through your cervix and out through your vagina. And what happens to the blood in your menstrual flow while it is sitting in the uterus?

It clots.

What Do Normal Period Blood Clots Look Like?

If your menstrual flow is of average volume it is possible that you may have some actual blood clots. These clots are usually quite small and typically dark red to almost black in color. The clots are small because there is only a moderate amount of blood lost that needs to clot. The clots are dark in color because the blood has been sitting in the uterus for awhile before it comes out in your menstrual flow.

When Period Blood Clots Might Be A Problem

If you are having a heavy menstrual flow your blood clots will appear very different. First of all these clots will be bigger because there is a larger amount of blood sitting in the uterus that needs to clot. Secondly, these clots are usually a much brighter red. This is because the blood is filling the cavity of the uterus faster. The pressure of the accumulating blood volume is moving the blood and clots out of the uterus at a faster rate.

Because with heavy bleeding the clots tend to be bigger you will likely also have more pain with your menstrual flow because the blood clots need to pass through your cervix. Your cervix will actually have to dilate or open a little to let the blood clots pass through.

The pain sensation associated with cervical dilation is cramping. The cramping can be quite intense and painful.  The bigger the blood clot the more the cervix has to dilate or open to let it pass and the more cramping you will have.

If you are having very heavy bleeding and passing clots the size of a grape it is important that you discuss this with your doctor.                                                                     

Depending on your age and your medical history your health care provider might suggest additional testing to try and find an underlying cause for your bleeding. Tests that your doctor may recommend include:

Common gynecological conditions that are associated with heavy vaginal bleeding and that might result in large blood clots include:

  • Uterine fibroids
  • Endometrial polyps
  • Adenomyosis

A Word From Verywell

Understanding your normal period is very important. Depending on your flow, small dark red blood clots may always be present or only sometimes present in your menstrual flow. This is usually completely normal.

If you have recently noticed an increase in your flow or amount and size of blood clots this could indicate and underlying condition and you should discuss it with your doctor. Also, if you have just started having your period and your flow is very heavy with larger blood clots this is not normal and you should tell your doctor about it.

Updated by Andrea Chisholm MD


Apgar BS, Kaufman AH, George-Nwogu U, Kittendorf A.(2007) Treatment of menorrhagia. American Family Physician, 75(12):1813–1819,1820.

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