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© Center for Endometriosis Care/Ken Sinervo MD, MSc, FRCSC. All rights reserved. No reproduction permitted without written permission. Revised since original publication and current as of 2023. No external funding was utilized in the creation of this material. The Center for Endometriosis Care neither endorses nor has affiliation with any resources cited herein. The following material is for informational purposes only and does not constitute medical advice.
By Robert B. Albee, Jr., MD FACOG ACGE FOUNDER
Menstrual clots are normal for many people with periods, but they can also signal changes that may or may not be significant. In this article, I'll try to explain the whys and wherefores of clots.
What Are They?
Clots are the natural result of your body taking care of you. They are a natural way of controlling bleeding. Scientifically, it is a complicated and involved process that leads to fibrin being formed into a matrix.
On a scraped knee, this mechanism leads to scab formation. But inside the uterus, the process results in a clot. Because blood in the uterus is mixed with many other cells and endometrial tissue, there is a difference between this clot and those seen on the skin.
Whenever blood pools or is retained inside the uterus it is likely to clot. When menstrual blood and cells pass immediately outside the body, clots are less likely. As a general rule, the redder the blood is, the faster it has reached the outside. The blacker it is, the longer it took to exit the body. If the blood is accumulating faster than the body’s ability to transfer it out of the uterus, clots are the result.
Are Clots Ever Normal?
Yes. Many patients routinely have some clotting during menses. Factors that determine whether clots will occur include:
the size of the uterus
the ability of the uterine myometrium (muscle) to contract
fibroids
adenomyosis
the diameter of the cervical canal through which the blood must flow
any obstruction to the outflow of menstrual products such as polyps, adhesions, etc.
How Should Clots be Evaluated?
The most important indicators are the blood count (hematocrit) and the duration of clotting. If a patient can maintain a normal blood count without developing anemia, and the clotting does not last more than the length of her normal period, it is usually considered within acceptable limits.
What Else Can it Mean?
When there is a significant change in the amount or duration of clotting, we should consider several possible explanations.
Pregnancy: When pregnancy is a possibility, it should always be tested for. Problem pregnancies, miscarriages, and ectopic pregnancies can all be associated with clotting.
Hormonal Changes:
Short term: Hormone production can vary for many reasons. Some of these include failure to ovulate, ovarian cysts, and medication. It is not uncommon for any patient to experience an occasionally bizarre menses with unusual clotting. The majority of these occurrences are short-lived and followed by normal periods.
Chronic: Other factors that can change hormone production on an ongoing basis include peri-menopausal changes, chronic illness, significant weight gain or loss, and use of certain long term medications such as steroids, or prolonged failure to ovulate.
Changes in the Uterus
Fibroids: called leiomyomata, fibroids are very common. They are benign growths of muscle that form spheres occupying space within the normal muscle of the uterine wall. They can grow into the inside of the uterus (submucous), stay within the wall (intramural), or grow from the outside surface (subserosal). Sometimes they grow on a stalk (pedunculated). If they interfere with the ability of the entire uterus to contract, bleeding will increase and clots are likely to form.
Adenomyosis: when this abnormality occurs, it is commonly associated with a reduction in the ability of the muscle fibers in the uterine wall to contract. This, in turn, can result in clotting.
Large Uterus: after multiple pregnancies or multiple births, the uterus may remain larger than it was pre-pregnancy. If the cavity of the uterus enlarges, more pooling of menstrual blood may occur. This results in an increase in clotting.
Polyps: polyps or other growths inside the uterus can act as an obstruction to the outflow of blood during menses. This can increase clotting. These growths can also bleed themselves.
Outflow Obstruction: anything that can partially block the outflow of fluid from the uterus during menses can contribute to clotting. The most common form of outflow obstruction results from natural changes after menopause when hormone levels drop. Without estrogen stimulation, the cervical canal may atrophy, or shrink. This can restrict the flow of blood, so clots are likely to form. This problem does not surface unless there is some reason for the post-menopausal patient to bleed. Then she may notice that the bleeding is not at all like her menstrual period used to be.
Does Endometriosis Cause Clotting?
In my experience about 30-40% of endometriosis patients experience abnormal bleeding, which may or may not include clotting. In general, I feel this is an indirect effect and not a direct one. For example, a patient who develops a large endometrioma may find that it interferes with normal ovulation. This could indirectly lead to abnormal bleeding and possibly influence clot formation.
In Summary…
Clots in and of themselves are generally not harmful. One menstrual cycle that is different from your normal pattern probably doesn’t mean anything. But if changes continue, you should bring them to the attention of your physician.