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

Many people 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 via a cascade of reactions between cells and protein molecules.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 so 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 enough time to act — so the blood clots.

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

Menstrual blood clots can be entirely normal. People 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.

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

It is also fairly common 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 these conditions can cause heavy menstrual bleeding.

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 what you would normally experience, then it may be indicative of something not being right.

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

Pregnancy

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).⁵ In this instance, it is important to seek medical advice.

Perimenopause

People who are perimenopausal experience fluctuations in oestrogen levels. When there is low oestrogen relative to progesterone levels, there is an 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, other symptoms may be present, such as hot flushes, palpitations, mood swings, and irregular or abnormal periods.

Bleeding disorders

People with bleeding disorders such as Von Willebrand disease (VWD), or platelet function defects, may find they have heavy periods with blood clots.⁶ 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.

Contraception

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 your period because this gives the womb lining time to build up — resulting in a heavier period when you do bleed.

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, there may also be pain in the pelvis or lower back, increased menstrual cramps, feeling like you 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 of adenomyosis, along 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.

Treatment

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.⁷ 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.⁸ These prevent the ovaries from releasing eggs in order to prevent pregnancy and therefore stop the body from having “natural” periods.

For people who have large fibroids, surgery may be a possibility. In others, 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.⁹ A thinner womb lining will mean less bleeding. The uterus lining can grow back, so some people 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 just to rule out anything serious. However, just 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 April 2022
Next update due 2025