Menstrual clots are lumps of coagulated blood or tissue. They are discharged from the uterus during menstruation when the lining of the uterus or endometrium sheds and is expelled from the uterus through the cervix.
Normally, the body produces substances known as anticoagulants  or blood thinners to allow the thickened endometrium fragments and blood mixture to pass more freely through the cervix and be expelled. However, sometimes, the body is not able to produce enough thinning agents, causing blood clots to form.
Menstrual clots resemble pieces of mashed up red fruit. They can be bright red or burgundy and may vary in size. They are usually mixed with liquid blood. The longer the blood stays inside the uterus, the darker it is in color, and the likelier it is to form clots.
Blood clot formation is the body’s way of controlling bleeding . Blood cells called platelets play an important role in blood clot formation. They release chemicals that start the so-called “clotting cascade.” A protein called fibrin is the last step in this cascade, which crosslinks and forms a mesh-like structure that results in a clot.
Platelets may be triggered to produce these clotting factors. This occurs during menstruation, as well as because of an injury to a blood vessel wall.
The normal menstrual cycle lasts 25 to 32 days. A normal flow lasts between four and seven days and may be accompanied by blood clots. A normal period should normally cause mild to moderate discomfort beginning the first day and then subside.
A period is considered heavy if it soaks through a pad or tampon every hour for several hours or if it lasts for over seven days . Large clots in period blood are also considered signs of heavy periods. Moreover, heavy periods can cause constant pain in the lower abdomen.
Heavy menstrual bleeding can negatively affect a woman's quality of life and lead to other conditions such as anemia  where the body lacks enough healthy red blood cells that can carry oxygen to different organs and tissues in the body. Anemia can make a person feel weak and tired.
Blood clots during a period can be normal or a sign of an underlying problem  such as:
Menstrual blood clots can also indicate the presence of more serious conditions such as:
Clots in menstrual blood are considered normal if they are quarter-sized or smaller and only occur occasionally. However, if they frequently occur (more then three times in a row) accompanied by heavy periods and are larger than a quarter, you should seek medical advice.
Endometriosis is usually accompanied by heavy bleeding during a woman’s period and chronic pelvic pain. This is because the endometrium that has grown outside the uterus reacts to the hormones secreted from the ovaries in the same way as the normal endometrium and starts to bleed.
Hormonal medications such as the contraceptive pill can help regulate heavy bleeding and reduce the formation of blood clots . These work by inhibiting the growth of the endometrium. An intrauterine device (IUD) that releases the hormone progestin can also help.
Other treatment options include Cyklokapron  and Lysteda . These medications stop blood clots from breaking down and therefore may reduce heavy menstruation. Likely less blood will be pooled in the uterus and menstrual clot formation will be reduced.
Endometriosis is associated with heavy periods, prolonged menstrual flow, and large clots. We believe that transvaginal sonogram, and, if needed, hysteroscopy and endometrial biopsy are necessary for diagnosis. It is also necessary to rule our the diagnoses of myoma, uterine fibroid, polyp, endometrial hyperplasia, and endometrial cancer.
"My research led me to locate an excision specialist, Dr. Seckin. I had excision surgery with Dr. Seckin in March of 2015. I had advanced endometriosis removed in 18 places, including my appendix, intestines, ovary, and uterus. It turns out that the cyst was the least of my problems! Dr. Seckin was wonderful throughout my experience." —M. Wilson
Medically reviewed by Tamer Seckin, MD on September 20, 2019
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