What Causes Menstrual Blood Clots?

Birth control pills can make a woman's period lighter and more tolerable.
A heavy menstrual flow with clotting could indicate a case of endometriosis.
A monthly period occurs when ovulation does not result in pregnancy, and the uterus sheds the lining is has built up.
A fibroid is a benign tumor that grows in the uterus, and may cause an increase in menstrual blood clots.
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  • Written By: Elizabeth West
  • Edited By: Allegra J. Lingo
  • Last Modified Date: 11 August 2014
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Menstrual blood clots can be caused by an imbalance in the clotting and anti-coagulating factors in blood or by hormonal changes. Fibroids and endometriosis contribute to excess buildup of the uterine lining, which breaks free as clots. Miscarriage can also be seen in thrombus form. Heavy menstrual flow should be evaluated by a doctor to ensure there are no serious problems.

When ovulation does not result in a pregnancy, the uterus sheds the lining it has built up for implantation. This process is menstruation, commonly called a period. Blood and tissue is expelled through the vagina. In women whose periods are heavy, blood clots may be seen. Although alarming, it is usually not a cause for concern.

The uterine lining secretes an anti-clotting factor during periods to assist the blood flow. If the substance is not sufficient to overcome the natural clotting factor menstrual blood clots can form, a process known as hemostasis. Flow may slow down toward the end of a period, and blood can clump. When women are reclining for a time, such as while sleeping, the discharge will pool and blood clots will be dislodged when upon standing. The blood darkens as it sits and sometimes looks brown or black.


A fibroid is a benign tumor that grows in the uterus. It may cause an increase in menstrual flow and menstrual blood clots. Fibroids usually have no symptoms, but large ones sometimes cause pain and heavy bleeding. They can be removed surgically or shrunk with medication. Most of the time they do not interfere with pregnancy.

Endometriosis is a condition in which the uterine lining, or endometrium, grows outside it, in the Fallopian tubes or abdomen. The tissue bleeds and sheds as it normally would, but since there is no exit for it, scarring and pain may occur. Heavy bleeding with clots or between periods is one symptom. The condition is treatable with medications and surgery. In severe cases, total hysterectomy along with removal of the ovaries may be the only solution.

A woman who is pregnant or may be pregnant and passing large menstrual blood clots may be experiencing a miscarriage. There is typically pain because the uterus contracts to expel the material. The placenta, lining, and other tissues may appear as large clots of blood or clumps of matter. Miscarriage usually occurs because something has gone wrong with the pregnancy and it is no longer viable. Older women and those who have had multiple pregnancies are more prone to miscarriage.

Hormonal changes may produce clots as well. If a woman has very heavy flow and persistent clots on a regular basis, she should consult her doctor. Birth control pills change the hormonal balance and may make periods lighter and more tolerable.


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