Why Do I Get Blood Clots During My Period

Noticing that you’re passing blood clots during menstruation (the period) can be a worrying experience, but this is usually a completely normal part of the period, especially if your blood flow is considered to be heavy. Blood clots passed during the period are more likely to occur during the heaviest days of the period, which can vary from person to person. People who have heavy periods are more likely to pass large blood clots, about the size of a 10p coin or 2.5cm in diameter. For most people, menstrual blood clots are a completely normal occurrence,  and it can cause problems such as bleeding through your menstrual products. However, it can,be a sign of another issue. There are other causes that create blood clots in menstrual blood which pass during the period. These which can be discussed with a doctor if you want a solution, but most are common and easy to treat.  

What are period blood clots

The bleeding during the period is the shedding of the uterine lining also known as the endometrium which thickens during the follicular phase of the menstrual cycle. During the process of the shedding, small blood vessels bleed and platelets are released to form blood clots to minimise the amount of blood lost. When there is a larger amount of bleeding, it collects in the uterus and stays there for too long. Essentially, menstrual blood clots are pieces of the uterine lining and blood which have coagulated together within the uterus or vagina before being passed. Menstrual blood clots tend to look like clumps of blood and can vary in size. They tend to become darker in colour as the period goes on, as they are made up of older blood. Clots usually form naturally after sitting or laying down for a long time, and they can be felt and passed once you’ve stood up. It is likely that more pain or cramping will happen when passing a menstrual blood clot, especially a larger one, as they cause the cervix to dilate more than usual. 

Causes of period blood clots

There are a few different causes for clots in period blood, some of these include: 

  • Birth control - When starting a new form of birth control, you are likely to see some changes in your menstrual cycle as your body gets used to the changes it causes. These changes can occur within a few days of beginning birth control, and they can be very different from person to person. Some people will find that their periods become lighter and less painful, and others may find that their periods are heavier and more painful. The frequency of periods can also change due to birth control. Birth control methods that may cause periods to become heavier, such as the non-hormonal IUD, can also increase the frequency of blood clot formation due to the larger amount of blood that can potentially coagulate.Medications - Some medications can change  the menstrual cycle, causing heavier periods and more frequent clotting. The most common types of medications that play a part in  the formation of abnormal menstrual clots are blood thinners, hormone medications, and anti-inflammatory medications. These can all increase abnormal flow, making periods heavier and creating more menstrual blood clots
  • Uterine Fibroids - Uterine fibroids are noncancerous growths that develop in and around the uterus. They are made up of muscle tissue and fibrous tissue. Most women do not experience symptoms caused by fibroids, so they are often unaware if they have any. However, fibroids can cause heavier, longer, and more painful periods, causing period blood clots in a similar way to birth control. There are different types of fibroids, but the most common fibroid that causes menstrual blood clots is the submucosal fibroid. This type of fibroid grows within the lining of the uterus and can cause clots to be much larger. They cause obstructions in the uterus, which makes it harder for the blood to pass and easier for large amounts of menstrual blood to settle and coagulate
  • Polycystic Ovary Syndrome - Polycystic Ovary Syndrome (PCOS) is a condition which affects how  the ovaries work. Itt is quite common and most women who have PCOS do not experience symptoms so they often don’t know that they have it. PCOS causes follicles to grow in the ovaries surrounding the eggs. They are often underdeveloped and the eggs are unable to be released, meaning that ovulation doesn’t take place. Because of this, PCOS causes hormonal imbalances which can contribute to changes in the menstrual cycle and menstruation. PCOS can cause very heavy and fast-flowing  periods, meaning that the natural anticoagulants (blood thinners) released by the body are unable to keep up. This results in more clots passing, as well as larger clots being formed

Management and treatment for period blood clots

As most clotting is caused by heavier periods, the management and treatment for menstrual blood clots involve  making periods lighter. Some types of contraception can be used to reduce the heaviness of periods, including the intrauterine system (IUS) or the combined pill. These work by making the uterine lining thinner, therefore reducing the amount of blood lost during the period. Medicines that help reduce the bleeding can also reduce the amount of blood clots that occur during the period. Tranexamic acid is one such medicine, and it is typically used to treat heavy bleeding during periods. It works by improving the blood clotting of the small blood vessels in the uterus, slowing down the flow of blood. It decreases the amount of blood lost during the period. The pain caused by menstrual blood clots can be managed in daily life using over-the-counter painkillers, similar  to other period pains.

When should I see a doctor

Although most menstrual blood clotting is a normal part of the period, it can sometimes be a sign of something underlying or a cause of pain and inconvenience. You should contact your doctor if clots  occur on a more frequent basis, if pain due to passing clots is not helped by over-the-counter painkillers, or if the clots become larger– about the size of or bigger than a 10 pence coin (2.5cm in diameter). 


Blood clots passed during the period are usually a completely normal and natural part of menstruation, and are often the result of sitting or laying down for a while or a part of a heavier period. However, they can be caused by other factors, and can indicate that medical intervention is needed. 


  1. Um, Should I Be Worried About Blood Clots During My Period? Women’s Health [Internet]. 2019 [cited 2023 May 31]. Available from: https://www.womenshealthmag.com/health/a19931429/clot-in-period-blood/.
  2. Beller FK. Observations on the clotting of menstrual blood and clot formation. American Journal of Obstetrics and Gynecology [Internet]. 1971 [cited 2023 May 31]; 111(4):535–46. Available from: https://www.sciencedirect.com/science/article/pii/0002937871904704.
  3. What you need to know about blood clots and menstruation. Mayo Clinic [Internet]. [cited 2023 May 31]. Available from: https://www.mayoclinic.org/diseases-conditions/menorrhagia/expert-answers/blood-clots-during-menstruation/faq-20058401.
  4. Santos IS, Minten GC, Valle NC, Tuerlinckx GC, Silva AB, Pereira GA, et al. Menstrual bleeding patterns: A community-based cross-sectional study among women aged 18-45 years in Southern Brazil. BMC Womens Health [Internet]. 2011 [cited 2023 May 31]; 11:26. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3118185/.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Jade Howlett

Bachelor of Psychology – BSc Psychology, University of Hull, England

Jade is a student at the University of Hull in her final year of her undergraduate course, with particular studies in Health Psychology. She will be pursuing a master’s degree in Artificial Intelligence and Data Science with the interest of combining the disciplines of Psychology and Artificial Intelligence.

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