Endometriosis

A long-term condition in which tissue similar to the lining of the womb grows outside it. It affects about one in ten women of reproductive age, and it is very often diagnosed late.

Overview

Endometriosis is a long-term condition in which tissue similar to the lining of the womb grows outside the womb. It most often grows on the ovaries, the fallopian tubes, and the tissue lining the pelvis. Less commonly, it is found on the bowel, the bladder, the ureters, the diaphragm or the chest.

This tissue behaves much like the womb lining. It thickens and bleeds with each menstrual cycle. But because it sits outside the womb, the blood has no way out. The result is repeated inflammation, scarring, and bands of scar tissue called adhesions that can stick organs together. On the ovary, the trapped blood can form a cyst known as an endometrioma.

Endometriosis is driven by oestrogen. Unusually, the growths also produce some of their own oestrogen, which is part of why the condition is so persistent.

It affects about one in ten women of reproductive age, roughly 190 million worldwide, according to the World Health Organization. It is not rare in African women, although it has long been described that way. A 2022 commentary in Reproduction & Fertility by Kenyan and UK researchers concluded that this belief stems from historical bias and low awareness among clinicians rather than from any biological difference, and that it has directly contributed to long diagnostic delays across the continent.

There is no cure. There are treatments that control it well, and they work better the earlier they are started.

Symptoms

The main symptom is pelvic pain, usually linked to the menstrual cycle. Many women have period cramps. In endometriosis, the pain is typically far worse, lasts longer, and tends to worsen over the years.

• Painful periods. Pain may begin a day or two before bleeding starts and continue for several days. It is often felt in the lower abdomen, lower back and thighs. The useful test is not how the pain feels but what it costs you: pain that keeps you from school, work or normal activity is not typical period pain.
• Pain during or after sex. Usually felt deep inside rather than at the entrance, and it may continue for hours afterwards.
• Pain when opening the bowels or passing urine. Most noticeable just before and during the period. Blood in the stool or urine at that time of the month is a warning sign.
• Bowel and stomach symptoms. Severe bloating, nausea, and alternating constipation and diarrhoea. These are often mistaken for irritable bowel syndrome, which can also exist alongside endometriosis.
• Heavy or irregular bleeding. Heavy periods or bleeding between periods.
• Fatigue. Persistent tiredness that is out of proportion to your activity.
• Difficulty getting pregnant. For some women, endometriosis is found for the first time during fertility investigations.
• Less common symptoms. Shoulder-tip pain, chest pain or breathlessness during periods, and pain running down one leg.

The amount of endometriosis in your body does not predict how much pain you will have. A small number of growths can cause severe pain. Extensive disease can cause little or none. Some women have no symptoms at all, and are diagnosed during fertility investigations or during surgery done for another reason.

When to see a doctor

See a gynaecologist if period pain regularly stops you from doing normal things, if pain is spreading beyond your period days, if sex is painful, or if you have been trying to conceive for 12 months without success.

Take a written symptom record with you. For two or three cycles, note your pain score out of ten, the days you missed work or school, the painkillers you took and whether they helped, and any pain with sex, stool or urine. This changes the consultation more than anything else you can do.

Causes

The exact cause is not known. Several mechanisms are thought to contribute, and more than one may be involved in the same person.

• Retrograde menstruation. Menstrual blood flows backwards along the fallopian tubes into the pelvis instead of leaving the body. Cells from the womb lining settle on pelvic surfaces and grow there. This happens in most women, so it cannot be the whole explanation.
• Cell transformation. Cells lining the abdomen, or leftover embryonic cells, may change into endometrial-like cells under the influence of hormones or inflammation.
• Spread through blood or lymph. Endometrial cells may be carried to distant sites, which helps explain disease in the lung or diaphragm.
• Surgical implantation. Cells may attach to a surgical scar after a caesarean section or other pelvic surgery.
• Immune failure. The immune system may not clear misplaced endometrial cells as it should.
• Progesterone resistance. The growths respond poorly to the body's own progesterone, allowing them to persist and spread.
• Genetics. Endometriosis runs in families, and large genetic studies have identified dozens of regions of the genome linked to the risk.

Risk factors

• Starting periods at an early age
• Short menstrual cycles, generally shorter than 27 days
• Heavy periods lasting longer than seven days
• Never having given birth
• Reaching menopause at a later age
• Higher lifetime exposure to oestrogen
• A mother, sister or aunt with endometriosis
• Low body mass index
• Any condition that blocks menstrual blood from leaving the body normally

None of these causes endometriosis on its own, and many women with the condition have no risk factors at all.

Complications

Infertility. Up to half of women with endometriosis have difficulty conceiving. Scarring can block the fallopian tubes, cysts can reduce the ovary's egg supply, and inflammation in the pelvis can interfere with the egg, the sperm and the early embryo. Many women with mild to moderate disease still conceive naturally.

Chronic pain. After years of untreated pain, the nervous system itself can become oversensitive. Pain then continues even after the growths are removed. This is one of the strongest arguments for early treatment.

Ovarian cysts and emergencies. Endometriomas can grow large, rupture, or twist the ovary, requiring urgent surgery.

Other health conditions. Endometriosis is linked to a higher likelihood of migraine, bowel conditions, anxiety and depression. A large observational study published in 2026 also reported an association with type 2 diabetes. Studies of this kind can show that two conditions occur together more often than expected, but cannot prove that one causes the other.

Cancer. Ovarian cancer is slightly more common in women with endometriosis. The starting risk is low and remains low. A rare cancer arising within endometriosis itself can occur later in life.

Diagnosis

• History and examination. A structured symptom history is the most valuable step. A pelvic examination may find tenderness, nodules or a uterus that does not move freely, but a normal examination does not rule the condition out.
• Ultrasound. A transvaginal scan reliably shows endometriomas and, in expert hands, deep disease. It cannot show the commonest form, superficial growths on the pelvic lining. A normal scan does not exclude endometriosis.
• MRI. Used to map deep disease before surgery, particularly involving bowel, bladder or ureter.
• Blood tests. CA-125 may be raised in advanced disease but is often normal in early disease and rises in many other conditions. It cannot confirm or exclude endometriosis.
• Laparoscopy. Keyhole surgery remains the only way to see and confirm the disease definitively. Since ACOG's February 2026 guidance, it is no longer required before treatment starts: a diagnosis based on symptoms and examination is enough to begin.

Treatment

Treatment depends on the severity of symptoms, whether pregnancy is wanted now, and what is available and affordable.

Pain relief. Two groups of medicines are used. Non-steroidal anti-inflammatory drugs (NSAIDs) — such as mefenamic acid, ibuprofen, naproxen or diclofenac — work by blocking the inflammatory chemicals that drive cramping. Simple analgesics such as paracetamol can be used alongside them. Timing matters more than most people realise: NSAIDs work best when started a day or two before bleeding is expected, rather than after the pain is established.

Take NSAIDs with food. Do not combine two different NSAIDs, and do not exceed the dose printed on the packet or prescribed for you. Tell your doctor before using them if you have stomach ulcers, kidney disease, uncontrolled high blood pressure or aspirin-sensitive asthma, or if you are pregnant or trying to conceive.

If you need painkillers at their maximum permitted dose every cycle and they still are not controlling your pain, that is a reason to return to your doctor rather than to keep increasing them. Opioid painkillers are not appropriate for long-term management of this condition.

Hormone treatment. The aim is to stop or reduce monthly bleeding, which quietens the growths. Options include the combined pill taken continuously without a break, progestin tablets such as dienogest or norethisterone, the depot injection, and the hormonal coil, which is particularly useful for heavy bleeding and for preventing recurrence after surgery. More powerful options, such as GnRH agonist injections given with protective add-back tablets, are reserved for severe disease. None of these improves fertility, and several prevent pregnancy while in use. All require a prescription and follow-up.

Surgery. Keyhole surgery to cut out the growths is used for deep disease, large cysts, or pain that has not responded to medication. It is effective but not permanent, so hormone treatment is usually continued afterwards. Removing the womb does not guarantee a cure, because the growths lie outside it.

Fertility care. Surgery can modestly improve natural conception in mild disease. IVF is often more effective in advanced disease. Raise fertility plans early.

Supportive care. Pelvic floor physiotherapy, heat, regular exercise and mental health support all have a role. Where pain has become long-standing and nerve-related, medicines that act on nerve pain rather than inflammation are sometimes added, under specialist guidance.

Pregnancy is not a treatment. Symptoms often ease during pregnancy and usually return afterwards.

Sources

This page summarises current guidance from the following. Readers and clinicians are encouraged to consult them directly.

• World Health Organization — Endometriosis fact sheet
• American College of Obstetricians and Gynecologists — Clinical Practice Guideline 11: Diagnosis of Endometriosis (February 2026)
• European Society of Human Reproduction and Embryology (ESHRE) — Guideline on the management of endometriosis
• National Institute for Health and Care Excellence (NICE) — NG73, Endometriosis: diagnosis and management
• Mecha EO, Njagi JN, Makunja RN, Omwandho COA, Saunders PTK, Horne AW. Endometriosis among African women. Reproduction & Fertility, 2022