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How endometriosis works

In short

Tissue like the lining of the womb grows outside the womb. The cause is not fully known. There is no cure. Treatment aims at symptoms.

Written by the OpenEndo team · Last reviewed · No separate clinical review yet

General information, not medical advice. How to use this site

How it develops

  • Established
  • Likely
  • Uncertain
  • Not yet shown

What the tissue is, and what is established about how it behaves. The cause itself is still unknown.

  1. 1

  2. 2

  3. 3

  4. 4

  5. 5

1 WHO fact sheet, 15 Oct 2025 · 2 ESHRE guideline, 2022 · 3 ESHRE patient leaflet, 2022

Last reviewed 28 Oct 2026

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  1. Similar tissue. Tissue like the lining of the womb grows outside the womb. (Established [1])
  2. Mostly the pelvis. It is most often in the pelvis. In some people it is in the abdomen or chest. (Established [1])
  3. Needs oestrogen. The tissue needs oestrogen, a hormone of the reproductive years, to establish and grow. (Established [2])
  4. Inflammation and scar. The tissue causes inflammation and scar tissue. (Established [1] [2])
  5. Cause unknown. Several theories exist. None is proven. (Established [1] [3])

Leading theories

  • Established
  • Likely
  • Uncertain
  • Not yet shown

Ideas about origin. The scale is a reading of the sources, not a percentage.

4 CMAJ review, 2023 · 5 Patienthåndbogen, sundhed.dk · 6 Cell review, 2021 · 7 Hum Reprod review, 2024 · 1 WHO fact sheet, 15 Oct 2025 · 3 ESHRE patient leaflet, 2022

Last reviewed 28 Oct 2026

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Retrograde menstruationLikelyBackward flow happens in about 90% of women who menstruate, so on its own it does not explain who gets endometriosis.
Coelomic metaplasiaUncertainIt could only explain lesions on the peritoneum (the lining of the abdomen), not lesions elsewhere.
Embryonic remnantsUncertainA proposal that leftover embryonic tissue becomes endometriosis. A 2024 review found low-certainty evidence and did not support it as the main route.
Blood or lymph spreadUncertainSuggested to explain disease outside the pelvis, including the chest. Not proven.
Stem cellsNot yet shownRare progenitor cells exist in the womb lining and in menstrual blood. It is not shown that they are the main starting tissue.
Genes and immunityLikelyIt runs in some families, and immune conditions are more common. These factors change risk. They are not a single cause.

What happens in the body

  • Established
  • Likely
  • Uncertain
  • Not yet shown

How the disease links to symptoms. How much pain you have does not follow how widespread the lesions are.

  • Established2

    • Established18

    • Likely9

    • Established105

    • Established1

  • Established1

    • Uncertain5

  • Uncertain24

2 ESHRE guideline, 2022 · 1 WHO fact sheet, 15 Oct 2025 · 4 CMAJ review, 2023 · 8 NHS, endometriosis · 3 ESHRE patient leaflet, 2022 · 9 NICE NG73, updated 2024 · 10 Lægehåndbogen, updated 12 Jun 2026 · 5 Patienthåndbogen, sundhed.dk

Last reviewed 28 Oct 2026

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  1. Oestrogen. Lesions depend on oestrogen. (Established [2])
  2. Inflammation. The tissue sets up inflammation. (Established [1] [2])
  3. Nerve changes. Ongoing pain can involve the nerves. The nervous system can become more sensitive to pain (central sensitisation). (Uncertain [2] [4])
  4. Lesions grow. Implants can grow and bleed with the cycle. (Established [2] [8])
  5. Scar and adhesions. Scar can stick organs together. These are adhesions. (Established [1] [3])
  6. Pain stays on. Pain can continue after lesions are treated. (Uncertain [4])
  7. Period pain. Pain with a period that stops daily life. (Established [1] [9])
  8. Pain with sex. Deep pain during or after sex. (Established [9] [1])
  9. Bowel or bladder. Deep disease can involve the bowel or bladder and cause pain there. (Established [10] [5])
  10. Getting pregnant. Some people have trouble conceiving. Many do not. (Likely [1])
  11. Fatigue. Severe pain, bleeding, and low iron can exhaust you. (Established [1])
  • Oestrogen → Lesions grow. Established [2]
  • Inflammation → Scar and adhesions. Established [1]
  • Nerve changes → Pain stays on. Uncertain [2] [4]
  • Lesions grow → Period pain. Established [1] [8]
  • Lesions grow → Pain with sex. Likely [9]
  • Lesions grow → Bowel or bladder. Established [10] [5]
  • Lesions grow → Fatigue. Established [1]
  • Scar and adhesions → Getting pregnant. Uncertain [5]

Diagnosis pathway

  • Established
  • Likely
  • Uncertain
  • Not yet shown

The NICE path in England. A normal scan does not rule endometriosis out. WHO puts the average delay at 4 to 12 years.

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  5. 5

9 NICE NG73, updated 2024 · 1 WHO fact sheet, 15 Oct 2025 · 4 CMAJ review, 2023

Last reviewed 28 Oct 2026

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  1. History and diary. Symptoms and a pain diary start the assessment. (Established [9] [1])
  2. Examination. An abdominal and pelvic exam is offered. A normal exam does not rule it out. (Established [9])
  3. Ultrasound or MRI. A transvaginal ultrasound can show cysts on the ovary and deep disease. It misses superficial disease. MRI is an option for deep disease. (Established [9] [4])
  4. Specialist care. Referral if first treatment fails, daily life is affected, or deep disease is suspected. (Established [9])
  5. Laparoscopy if needed. Keyhole surgery can be considered even when scans are normal. Treatment can start before surgery. (Established [9] [1])
  6. Normal scan?. It can still be endometriosis. Next step: referral can still be right. (Established [9])

Treatment options

  • Established
  • Likely
  • Uncertain
  • Not yet shown

Options the guidelines name. They are not ranked. There is no cure, and no doses are given here.

Pain reliefUncertainUncertain
HormonalEstablishedEstablishedEstablishedLikelyLikely
SurgeryLikelyLikelyUncertain
FertilityEstablishedLikelyLikely
SupportUncertain

9 NICE NG73, updated 2024 · 2 ESHRE guideline, 2022 · 1 WHO fact sheet, 15 Oct 2025

Last reviewed 28 Oct 2026

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  1. Paracetamol or NSAID. A short trial can be considered for pain, and these medicines may be offered. The trials behind that are very low quality. (Uncertain [9] [2])
  2. TENS. Small trials found some improvement in pain or quality of life. They are low quality, and ESHRE makes no recommendation. (Uncertain [2])
  3. Combined pill. It reduces pain with periods, pain with sex, and pelvic pain. It does not permanently reduce later fertility. It is not used to help you conceive. (Established [9] [2] [1])
  4. Progestogens. Progestogens reduce endometriosis pain. Side effects differ between products. (Established [9] [2])
  5. Hormonal coil. A levonorgestrel coil reduces endometriosis pain. It is also a contraceptive. (Established [2] [1])
  6. GnRH agonist. Second line, because of side effects from low oestrogen. Add-back treatment (low-dose oestrogen and progestogen) is used to protect bone and limit those effects. (Likely [2] [1])
  7. GnRH antagonist. This can be considered. The recommendation is weak, and it is second line. Add-back treatment (low-dose oestrogen and progestogen) limits low-oestrogen effects. (Likely [2] [9])
  8. Excision or ablation. Surgery is one option to reduce pain. A Cochrane review cited for this is uncertain about the size of the effect on overall pain. Lesions can return. (Likely [9] [2] [1])
  9. Ovarian cyst surgery. Removing the cyst wall (cystectomy) is preferred to reduce pain and recurrence. Draining and ablating it may spare more ovarian reserve when fertility is a priority. (Likely [9] [2])
  10. Hysterectomy. Removal of the womb is not a cure. It is considered when other care has not helped and pregnancy is not planned. Visible lesions should be removed at the same time. (Uncertain [1] [2] [9])
  11. Hormones and conception. Hormonal treatment is not offered to raise the chance of conceiving without assistance. (Established [9])
  12. Surgery and conception. Removing peritoneal disease, or treating an endometrioma, can improve the chance of conceiving without IVF. (Likely [9])
  13. Fertility treatment. Ovulation induction, insemination, or IVF may be offered. Among women with infertility, as many as 25 to 50 percent have endometriosis. (Likely [1])
  14. Physio and psychology. Physiotherapy and cognitive behavioural therapy can reduce pain for some people. No specific non-medical treatment is recommended, because benefits and harms are unclear. (Uncertain [1] [2])

Known and not yet known

  • Established
  • Likely
  • Uncertain
  • Not yet shown

A short account of what the sources support, and what they still list as open.

Known

Not yet known

  • Which lesions will grow, stay the same, or shrink is not predictable for one person.
  • Treatments that control pain without blocking pregnancy are still needed.

1 WHO fact sheet, 15 Oct 2025 · 2 ESHRE guideline, 2022 · 9 NICE NG73, updated 2024 · 3 ESHRE patient leaflet, 2022 · 4 CMAJ review, 2023

Last reviewed 28 Oct 2026

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  1. About 1 in 10. WHO estimates about 10 percent (190 million) of women of reproductive age. ESHRE says the exact figure is unknown and cites a range of 2 to 10 percent. (Likely [1] [2])
  2. No cure. Treatment aims to control symptoms. Symptoms can continue or return. (Established [1] [2])
  3. No blood test. Do not use CA125 to diagnose it. No biomarker is proven for diagnosis. (Established [9] [3])
  4. Diagnosis takes years. WHO says the average time to diagnosis is 4 to 12 years. (Established [1])
  5. The cause. The cause is not known. The theories above are not settled. (Established [1] [3])
  6. A simple test. Saliva, blood, and menstrual-blood tests are described as emerging. They are not the established way to diagnose. (Not yet shown [1])
  7. Prevention. There is no known way to prevent endometriosis. (Established [1])
  8. Who relapses. Lesions can return after removal, and symptoms often return when treatment stops. A 2023 review cites about 22 percent at 2 years and 40 to 50 percent at 5 years after surgery. (Uncertain [1] [2] [4])

Sources

  1. Fact sheetEndometriosis fact sheet. World Health Organization. Key facts; Overview; Causes; Diagnosis; Treatment; Impact; Prevention; Challenges. 28 Oct 2026
  2. GuidelineESHRE guideline: endometriosis (2022). ESHRE. Introduction; I. Diagnosis; II. Treatment of endometriosis-associated pain, recommendations 10–38. 28 Oct 2026
  3. GuidelineInformation for women with endometriosis. ESHRE. What causes endometriosis?; Symptoms and diagnosis; staging. 28 Oct 2026
  4. ReviewDiagnosis and management of endometriosis. Canadian Medical Association Journal. What causes endometriosis?; subtypes; central sensitization; surgery recurrence. 28 Oct 2026
  5. Fact sheetEndometriose. Patienthåndbogen, sundhed.dk. Hvorfor får man endometriose?; symptoms from bowel, bladder, and adhesions. 28 Oct 2026
  6. ReviewEndometriosis: etiology, pathobiology, and therapeutic prospects. Cell. Endometriotic lesions: Origins; Genetic and genomic impacts. 28 Oct 2026
  7. Systematic reviewMüllerian anomalies and endometriosis as models for retrograde menstruation and embryonic-remnant theories. Human Reproduction. Abstract; Wider implications. 28 Oct 2026
  8. Fact sheetEndometriosis. NHS. Symptoms; Non-urgent advice: See a GP. 28 Oct 2026
  9. GuidelineEndometriosis: diagnosis and management (NG73). NICE. Recommendations 1.3 to 1.10, and the November 2024 and April 2024 update notes. 28 Oct 2026
  10. GuidelineEndometriose. Lægehåndbogen, sundhed.dk. Resumé; Behandling; Henvisning. 28 Oct 2026