Reviewed June 2019

When a menstrual blood clot could be a sign of something more

menstrual blood clot

Many women who have periods will be familiar with menstrual clots — clumps of blood cells and protein strands called fibrin, which come out with your menstrual blood. These clots can be completely normal, but occasionally they might be a sign of a problem. By looking at what causes menstrual clots, and symptoms you can look out for which may be a sign of something else, this article should help you understand what’s normal, and what might need checking out.

What is a menstrual clot?

Menstrual clots are a natural way for the human body to control bleeding, as it is the body’s natural reaction to try to form clots in order to stop us from bleeding excessively. Whenever blood pools in the uterus, the menstrual blood tries to clot by a cascade of reactions between cells and protein molecules, then a clot forms.1

This clot is made up of blood cells, fibrin, and endometrial tissue. It is like a scab that forms on the skin when the skin is cut and the blood clots on the surface. In this case, the “scab” is the blood clot forming inside the uterus or cervical canal.

At the same time, anticoagulants (proteins that stop the blood from clotting) are released to try to stop the blood from clotting and to break down uterine issue. This is the body’s way of maintaining a balance in order that the blood does not all clot in the uterus and is thin enough to come out of the womb. If menstrual blood passes out of the uterus quickly then there is no time for anticoagulants to act, but this doesn’t matter as there is no time for clots to form. However, if bleeding is heavy, clots have time to form, but the anticoagulants still do not have time to act — so the blood clots.

What’s normal when it comes to menstrual blood clots?

Menstrual blood clots can be entirely normal. Women who are more likely to have clots tend to have larger wombs and a narrow diameter of cervical canal, meaning blood is more likely to pool.

Women who have womb muscles that contract less well, or less forcefully, would be less able to stop bleeding because contraction of the womb muscle is important in order to “clamp” down on the blood vessels and reduce menstrual bleeding.2 Therefore these women will have more menstrual bleeding, less time for anticoagulants to act, and therefore be more likely to experience clots.

It is also fairly usual to present with clots if you have fibroids (non-cancerous growths of the womb tissue), polyps (outgrowths of womb tissue on a stalk), or adenomyosis (inner lining of the womb breaks through into the muscle layer of the uterus),3 because menstrual bleeding is heavy and blood is formed faster than anticoagulants have time to act, so the blood clots. These conditions are discussed in further detail below.

When menstrual clots occur, it most noticeable on the heaviest days of your period. For many, this is the initial few days. In general, blood is more red if the blood comes out of the womb rapidly, while a darker colour is associated with it staying in the womb for longer, or a slower flow of menstrual blood.

There is no such thing as a “normal” size of clot, but in general if it’s bigger than the size of a 50 pence piece or if the clots are bigger than normal or what you normally experience, then it may be indicative of something not being right.

Are there any health conditions that can cause menstrual blood clots?


If pregnancy is a possibility, it is important to carry out a pregnancy test, because heavy bleeding associated with greyish clots could be due to miscarriage.4 If there is severe pelvic pain, heavy periods with blood clots, and the pregnancy test is positive this may be due to an ectopic pregnancy (pregnancy occurring outside of the womb). In this instance it is important to seek medical advice.


In women who are perimenopausal, there are huge fluctuations in oestrogen levels. When there is low oestrogen relative to progesterone levels, there is excessive thickening of the womb lining, which can lead to heavy periods when the lining sheds. As explained previously, heavy periods do not give time for anticoagulants to act, and so blood clots may occur. In perimenopause, women may have other symptoms such as hot flushes, palpitations, mood swings, and irregular or abnormal periods.

Bleeding disorders

Women with bleeding disorders such as Von Willebrand disease (VWD), or platelet function defects, may find they have heavy periods with blood clots.5 This is because the body has an inability to stop bleeding due to abnormal blood cells or loss of anticoagulants. In these conditions, there may be bleeding elsewhere in the body such as frequent nose bleeds and difficulty in stopping bleeding after dental procedures.


Contraceptive pills do not generally cause heavy periods, but using copper IUDs or having the Depo-Provera injection can cause longer menstrual flows and heavier flows because contraction of the womb may be affected. As previously mentioned, if flows are heavy, blood clots may be present.

Similarly, skipping withdrawal bleeds when you are on the combined oral contraceptive pill can also cause menstrual clots when you do eventually have period, because this period may be particularly heavy.

Conditions associated with heavy menstrual bleeding

Any condition that is associated with heavy menstrual bleeding may be associated with menstrual blood clots. Fibroids are benign growths of the womb muscle that can interfere with how the womb contracts. Because of this, bleeding increases and clots are more likely to form. With fibroids, women may also get pain in the pelvis or lower back, increased menstrual cramps, the feeling that they need to go to the toilet more frequently, and pain during sex.

Adenomyosis is where the inner lining of the womb breaks through into the muscle layer of the uterus. Like fibroids, in adenomyosis, the uterine muscles do not contract well, periods are heavy, and blood clots are likely to form. Heavy periods are a key feature with spotting between periods, longer menstrual cramps and/or menstrual cycle, lower abdominal pain, pain during intercourse and increased risk of miscarriage.

The clots may not be different between these various conditions, but signs that clots may be abnormal include larger clots, multiple clots, menstrual clots with every period, heavy periods, pelvic pain, bleeding in other parts of the body, and infertility. If you experience any of these symptoms it is important to seek advice from your GP.


When menstruation is heavy, the blood flows too quickly for the anticoagulants to function correctly and menstrual clots form, which is why you are more likely to get blood clots if you have heavy periods. If your periods are so heavy as to cause distress, it is worth visiting your GP as certain medications may help.

Tranexamic acid, for example, is a drug that helps blood to clot in the womb and is taken three times a day for the first few days of the period. It doesn’t affect fertility.6 Non-steroidal anti-inflammatory drugs (NSAIDS) such as mefenamic acid or naproxen may also reduce heavy periods.

For some people the combined or mini contraceptive pill/Mirena coil may be used to help control heavy periods, regulate the menstrual cycle, and reduce period pain. These stop the ovaries from releasing eggs in order to prevent pregnancy and stops the body having “natural” periods.7

In women who have large fibroids, surgery may be a possibility. In other women, procedures such as endometrial ablation, where a probe is inserted into the womb and the use of microwaves or heat are used to thin the uterus lining, can reduce heavy periods.8 A thinner womb lining will mean less bleeding. The uterus lining can grow back, so some women can still have heavy bleeding after the procedure. But, in most, the hope is that the womb lining does not grow back to be so thick as to cause heavy bleeding.

If you are concerned about the size or amount of menstrual blood clots you are experiencing, it is worth going to your GP jut to rule out anything serious. However, for many women, seeing occasional clots, during one or two of your periods, is perfectly normal and healthy.
Featured image is a person with a vagina sitting on a sofa, clutching a hot water bottle to their pelvis area. The hot water bottle has a red tartan cover
Last updated June 2019
Next update due 2022

Dr. Diana Chiu, MBChB (Hons) MRCP PGCERT (Med Ed) PhD

Diana received her medical degree, with honours, from the University of Manchester. She then went on to receive basic and specialist medical training within the north west of England. She carried out in-depth research in medicine and was awarded a PhD in 2016. Currently, she is finishing her medical training at a large teaching hospital, and one of her greatest interests is medical education. She is an advanced life support instructor and writes regularly for post-graduate examination websites, and also holds a PGCERT in medical education with distinction.

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  1. Hapangama, D.K., and Bulmer, J.N., Pathophysiology of heavy menstrual bleeding, Women’s Health, January 2016, accessed online
  2. Institute for Quality and Efficiency in Health Care, Heavy periods: Overview,, [website], (accessed 28 June 2019)
  3. Mellor, Y., Heavy menstrual bleeding: diagnosis and management options, The Pharmaceutical Journal, June 2018, [website], (accessed 27 June 2019)
  4. D’ippolito, S., et al., The chromosome analysis of the miscarriage tissue. Miscarried embryo/fetal crown rump length (CRL) measurement: a practical use, PLOS One, June 2017, [published online]: (accessed 28 June 2019)
  5. Bevan, J.A., et al., Bleeding disorders: a common cause of menorrhagia in adolescents, The Journal of Pediatrics, June 2001, vol 138, issue 6, pp 856-861
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  7. NICE, Menorrhagia, Scenario: Management of menorrhagia, last revised December 2018, [website],!scenario (accessed 28 June 2019)
  8. Kumar, V., et al., Endometrial ablation for heavy menstrual bleeding, Women’s Health, January 206, vol 12, issue 1, pp 45-53

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