What is endometriosis?

Endometriosis is a chronic condition in which tissue similar to the lining of the uterus (endometrium) is found outside the uterus, most often inside the pelvis (ovaries, tubes, the lining of the abdomen). It most commonly causes pain and difficulty getting pregnant, although not everyone has symptoms. There is no known cure, but there are options to reduce symptoms.

How common is endometriosis?

The exact figure is unknown because diagnosis is difficult and slow. According to the 2022 ESHRE guideline, an estimated 2% to 10% of the general female population is affected, and the share may be up to 50% among women with fertility problems. From these estimates, at least 190 million people of reproductive age are thought to be affected worldwide.

The disease is fed by estrogen, so it is mostly seen during the reproductive years, but its effects can continue until and after menopause.

What causes endometriosis?

The exact cause is unknown. The most discussed theories are:

• Some menstrual blood flowing backward through the tubes into the abdominal cavity (retrograde menstruation).
• Family predisposition; it is seen more often in people with a close relative who has it.
• Immune system and hormonal factors.

No single theory explains every case. What matters: endometriosis is not caused by anything you did or did not do.

What are the types of endometriosis?

The ESHRE guideline describes three main presentations:

Type	Where it is found	In short
Superficial peritoneal	Surface of the abdominal lining	The most common appearance; often not visible on imaging
Ovarian endometrioma	Ovary	Popularly called a “chocolate cyst”, see our chocolate cyst article
Deep endometriosis	Lesions growing beneath the lining; organs such as the bowel and bladder	Can cause more severe symptoms; assessment at a specialist centre is recommended

Deep endometriosis involving the bowel has been reported in roughly 5% to 12% of affected patients in some studies.

What do the stages of endometriosis mean?

The disease is often staged I to IV with the rASRM classification. However, the ESHRE guideline states there is no convincing correlation between this stage and the severity of symptoms. A low-stage disease can be very painful and a high-stage disease can cause few symptoms. The NICE guideline also advises planning treatment around a person’s symptoms, priorities and preferences rather than the stage.

Does endometriosis progress over time?

It does not follow the same course in everyone. In trials of medical or surgical treatment, among women in the placebo or sham arm, about 29% had a higher disease score after 3 to 6 months, 29% had no change and 42% had a lower score (data reported in the 2022 ESHRE guideline). These figures come from small groups and should be read with caution.

What can you do if you have symptoms?

Recognising symptoms is the first step. You can read our symptoms page and fill in the symptom checklist to prepare a summary to take to your doctor. If you are curious about the diagnostic path, see the diagnosis page.