What are the causes of menstrual blood clots?

Updated November 21, 2016

If you are menstruating and pass a big blood clot, it can be scary. However, for the most part, passing blood clots while menstruating is not considered dangerous or necessarily an indication of trouble. According to Dr. Robert B. Albee of (see Resources), if blood pools for a while within the uterus, it will clot.

The Heavier You Bleed ...

If your menstrual periods are typically light, you probably won't have issues with clotting; however, if you tend to be a heavy bleeder, you will. According to (see first Reference), all blood contains a clotting factor; however, in order for your menstrual blood to flow freely and out of your body, the uterus produces an anti-clotting agent. If your flow is extremely heavy, the anti-clotting agent can be used up before your period is finished and, at that point, the blood that remains will begin to clot.


The Developmental Disabilities Health Alliance (see second Reference) explains that clots are clumps of blood that have pooled in the vagina. If you are lying down and then suddenly get up, that's when a clump of blood, or a clot, is likely to release from the vagina. (ee third References) notes that blood clots are simply the shedding of the uterine lining and are no cause for alarm. points out that blood clots can be the result of a woman's hormones being in flux. When the hormones are working exactly as they should, a woman's menstrual period shouldn't be excessive and she isn't apt to pass clots, which can be produced from hormonal imbalance. The wall of the uterus, from which the lining is shed each month during the menstrual period, can get thick if the hormones are out of control (too much oestrogen, too little progesterone or vice versa), and this often results in clots. notes that fibroid tumours, which are benign and which many women have, can cause clotting to occur. An enlarged uterus may result in clotting as well.


Endometriosis is a painful condition, which causes numerous unpleasant symptoms, including the passing of blood clots. According to (see Resources), the endometrium, which is the lining in the womb, can grow in the wrong place, including on the Fallopian tubes, on the ligaments that support the woman, between the womb and the bowl, and other places in the pelvic region. When this occurs, a woman is suffering from endometriosis. This lining, even when it is in the wrong place in a woman's body, reacts to the hormonal cycle and will shed blood each month. However, the blood that is attached to organs outside of the uterus has nowhere to go and will stay in the abdominal cavity. This can result in scar tissue developing, adhesions, inflammation and pain.


According to Dr. JoAnn Manson, a professor of Medicine at Harvard Medical School and chief of Preventive Medicine at Brigham and Women's Hospital, on (see Resources), if you are consistently having heavy menstrual periods that include lots of blood clots, you should mention this to your physician. Dr. Manson notes that this can be related to the beginnings of menopause, but you may also be experiencing anaemia (iron-poor blood) due to the excessive loss of blood each month. Anemia occurs when there is a deficiency in your red blood count, and can occur if you bleed too much. In addition, if you have never passed blood clots before, you may want to address this with your physician. It might or might not be a cause for concern.

Colour of the Blood

Dr. Albee (See Additional Resources) notes that the redder blood is, including menstrual blood, the faster it has reached the outside of your body. If the blood is dark red, it has been in the uterus for a while. If blood is accumulating faster than the body is able to rid itself of it, clots will occur.


If you are passing large blood blots and there is a possibility that you could be pregnant, you may be miscarrying and need to contact your physician. If you have miscarried, you may require a D&C procedure.

Young Women

If you are a young woman and still a novice when it comes to menstrual periods, it may frighten you to see a blood clot for the first time. If you pass just a couple of blood clots, there is no reason to be alarmed; however, if you are passing clots all day long and bleeding excessively as well, contact your physician. She can check you out and assure that all is well or, conversely, determine if there is a problem that needs to be addressed.

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About the Author

Cindi Pearce is a graduate of Ohio University, where she received her bachelor’s degree in journalism. She completed both the undergraduate and graduate courses offered by the Institute of Children’s Literature. Pearce has been writing professionally for over 30 years.