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pelvic pain in women

What causes endometriosis?

Endometriosis is a common long-term condition. In endometriosis, cells that are like the lining of the womb (uterus) grow outside the womb, such as on other reproductive organs like the Fallopian tubes and the ovaries.

For the purpose of this article 'girls, females, or women' are those individuals assigned as female at birth. It is not intended to exclude or dismiss individuals who do not identify as female.

Endometriosis is caused by tissue similar to the lining of the womb which attaches to, and grows on, other parts of the body outside the womb. Endometriosis tissue is usually found on the outside of the ovaries, the outside of the womb, and the ligaments inside the pelvis which keep the organs in place. It can also be found on the lining of the abdominal and pelvic cavity - the area around all of the organs inside the tummy. In rare cases, it can grow elsewhere in the body, like in the chest or in other organs.

Like the normal lining of the womb, endometriosis tissue thickens and bleeds in response to the normal hormones that control the menstrual cycle. In endometriosis tissue, the blood cannot easily leave the body, and it causes inflammation and scarring. It can also cause scar-like tissue to form between organs - called adhesions. Endometriosis tissue can also lead to cysts - fluid-filled lumps - called endometriomas. All of these can lead to pain and any other symptom of endometriosis.

We don't know exactly why some women develop endometriosis. Some of the leading theories for the causes of endometriosis include:

  • Problems with menstrual - period - blood flow that cause cells from the lining of the womb to enter the pelvic cavity, rather than leaving with menstrual blood.

  • Endometriosis-causing cells that spread, from the womb, through blood vessels or lymph vessels, to other parts of the body.

  • Problems with the immune system, meaning it doesn't detect and destroy endometriosis-causing cells that are in the wrong part of the body.

  • Genetic problems - endometriosis tends to run in families and some things leading to endometriosis might be inherited in genes.

  • Cells that are already present in other body areas might be able to transform into endometriosis tissue, under some circumstances.

  • Oestrogen seems to be important in the development of endometriosis, so hormonal problems could be linked to endometriosis.

  • Toxins in the environment - but there's little evidence to show this affects humans.

  • Problems with pain sensation. Like in any chronic pain condition, the way in which the body and the brain process and produce the sensation of pain can produce severe and difficult-to-control pain.

Like many conditions, endometriosis is very complicated, and there's probably no one single explanation for why it develops. It's more likely to be a combination of things and this might vary from person to person.

If you think you may have, endometriosis find out what to do next by clicking here. This will tell you if you need to see a doctor and how it is treated.

In this series of articles centred around endometriosis you can read about endometriosis symptoms, endometriosis treatment, and endometriosis causes - all written by one of our expert GPs.

The rest of this feature will take an in-depth look at the causes of endometriosis as, at Patient, we know our readers sometimes want to have a deep dive into certain topics.

Continue reading below

Causes of endometriosis

Endometriosis occurs when endometriosis tissue - tissue that is similar to the lining of the womb - grows outside of your womb (uterus). There are several theories as to how this tissue gets there.

Menstrual blood flow problems

Problems with menstrual blood flow is one of the leading theories as to why endometriosis develops. During a normal menstrual cycle, the lining of the womb bleeds and sheds, coming out through the vagina as menstrual bleeding. Cells from the lining of the womb can also go in the other direction, passing up the fallopian tubes and out into the pelvic cavity - the area surrounding all of the abdominal and pelvic organs - this is called retrograde menstruation. It's thought that some of these cells can survive and attach to the lining of the pelvic cavity, growing into endometriosis tissue.

However, this alone doesn't fully explain endometriosis. Retrograde menstruation happens in most women - but doesn't usually lead to endometriosis. This theory also can't explain how endometriosis can develop in other body parts, like the chest or other organs.

Spread through blood or lymph vessels

Another theory is that cells that can develop into endometriosis might be able to get into blood vessels or lymph vessels - lymph vessels drain fluid, called lymph, from the body's tissues - and spread to other body areas. This would explain how endometriosis tissue can, rarely, grow in other areas of the body - like around the lungs.

Transformation of other cells

It's been suggested that some cells which are already outside the womb could transform into endometriosis tissue under certain conditions. For example, cells on the lining of the abdominal cavity - the inside of the abdomen - can turn into endometriosis cells under certain circumstances. These cells originally develop from the same 'ancestor' as womb lining cells when embryos are developing in the womb - this is called the coelomic metaplasia theory. Metaplasia is a process where one type of cell turns into a different type.

Immune system problems

In endometriosis, there might be problems with the immune system which mean it doesn't detect and destroy endometriosis cells that have got into the wrong place. People with endometriosis have a higher risk of some autoimmune conditions, such as rheumatoid arthritis and systemic lupus erythematosus - although it's not known why this is the case.

Areas of endometriosis lead to inflammation, which is the immune system's response to damage. Whilst inflammation is good for many things - like fighting off infections or repairing an injury - it might actually make endometriosis worse, such as by causing endometriosis tissue to grow, or causing scarring.

It's also not known if changes in the immune system seen in women with endometriosis are the cause of endometriosis, or are caused by endometriosis itself.

Genetic problems

Some families have a strong history of endometriosis, which suggests that it could be passed on through genes. Some genes have been found that are linked with endometriosis, but there isn't a full understanding on the link between genetics and endometriosis. An endometriosis gene that clearly causes the condition, has not been found. It's more likely that there are several different genes that could make people more vulnerable to other causes of endometriosis.

Hormone problems

Oestrogen, a naturally-produced hormone, causes endometriosis to grow and spread. This explains why endometriosis is rare in girls who haven't gone through puberty, and why it tends to get much better after menopause - in both cases, oestrogen levels are low. Imbalances in hormone levels could contribute to endometriosis, and changes in endometriosis tissue might cause it to be extra-sensitive to oestrogen, even if body levels are normal. Hormonal therapy - including contraceptives/birth control pills - is one of the ways to treat endometriosis.


Various theories suggest that exposure to toxins in the environment could cause endometriosis. Studies have shown a link between dioxins - a type of pollutant - and endometriosis in animals but there's not enough evidence to show if this happens in humans.

Problems with pain sensation

It is still not fully understood why endometriosis leads to pain - especially how it causes the long-lasting (chronic) severe pain that some women experience. It's thought that endometriosis pain might be due to one or more of the following:

  • Pain felt because of the inflammation caused by endometriosis tissue - this is called nociceptive pain and is the type of pain we experience normally from any injury.

  • Pain caused by damage to, or changes in, the nerves - for example, new pain-sensing nerves might grow near areas of endometriosis, or other nerves might behave differently and start sending pain signals. This is called neuropathic pain, meaning it is caused by a nerve problem.

  • Pain caused by problems in the way the nervous system, probably the brain and spine, is processing signals. This could be that the brain might become sensitised to pain, so that low-level signals that would normally not be noticed, instead cause pain. This is called nociplastic pain, and is complex, and not well understood. It can be an important part of chronic pain conditions.

Women with endometriosis might experience one or more of these types of pain. This might change over time, and differs from person to person.

Patient picks for Periods and period problems

Is endometriosis caused by stress?

Women with endometriosis often experience high levels of stress, as well as mental health issues like depression and anxiety. Stress and endometriosis are linked, but stress isn't a cause of endometriosis, though it can affect how people experience the condition.

Endometriosis can be a very painful and debilitating condition and this can cause significant emotional and psychological distress, including depression and anxiety.

The experience of pain can also be influenced by stress and emotion. Stress can cause the brain to become more sensitive to pain, in ways that are complex and not fully understood.

So, endometriosis can cause low mood, stress, and anxiety - all of which can make pain more severe, so symptoms become more intense.

Untangling this relationship can be tricky. Many women with endometriosis have had their symptoms dismissed - for example, they've been told their symptoms are exaggerated, imagined, or due to a 'low pain threshold'. Of course, this isn't true - the symptoms are real. It's instead important to recognise stress, anxiety, and depression in women with endometriosis, and ensure that these are treated, alongside the endometriosis symptoms.

Continue reading below

How common is endometriosis?

Endometriosis is common and is estimated to affect one in every 10 women of reproductive age - in the UK, around 1.5 million women have it.

Endometriosis risk factors

Several risk factors have been identified for endometriosis. These include:

  • Starting periods at an early age - before the age of 11.

  • Going through menopause at a later age.

  • Having children later in life, or not having children at all.

  • Having shorter menstrual cycles - less than 27 days.

  • Having heavy periods.

  • Being born with a blockage of the vagina that prevents mucus and menstrual blood from leaving the vagina, such as an imperforate hymen that completely blocks the vaginal entrance.

Continue reading below

How to prevent endometriosis

Unfortunately, there aren't any reliable ways to prevent endometriosis from occurring. Trying to diagnose endometriosis early and treat it sooner is helpful. Although diagnosing endometriosis can be difficult, because the symptoms can be very varied, and might also be caused by many other conditions.

Some people have suggested that doing things to control oestrogen levels might help to reduce the risk of endometriosis occurring - for example, exercising regularly, maintaining a healthy weight, and avoiding excess alcohol. All of these things are good for overall health, but it hasn't been shown that they prevent endometriosis.

Hormonal treatment with contraceptives also helps to control oestrogen levels, and is a good treatment for endometriosis. However, it hasn't been proven it prevents endometriosis from happening.

Article history

The information on this page is peer reviewed by qualified clinicians.

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