Endometriosis UK: Symptoms, Causes & the Diagnostic Delay

A clinically reviewed UK guide to endometriosis — what it is, why it is so commonly delayed in diagnosis, symptoms, types including deep infiltrating disease, and the treatment pathway from pain relief to surgery.

What Is Endometriosis?

Endometriosis is a condition in which tissue that is histologically similar to the endometrium (the inner lining of the uterus) grows at sites outside the uterus. These deposits of ectopic endometrial tissue — called endometriotic lesions or implants — can be found on the ovaries, fallopian tubes, pelvic peritoneum, bowel, bladder, rectovaginal septum, and, rarely, at distant sites.

Like the normal endometrium, endometriotic lesions respond to hormonal fluctuations during the menstrual cycle. Each month, the lesions undergo similar proliferation, breakdown, and attempted shedding. Unlike the endometrium, this blood and tissue has no route out of the body, causing local inflammation, scarring (adhesions), and the formation of endometriomas (blood-filled ovarian cysts sometimes called “chocolate cysts”).

The result is a cycle of inflammation, scarring, and chronic pain that can worsen progressively over years if untreated — and that has profound consequences for quality of life, fertility, and general health.

How Common Is Endometriosis?

Endometriosis affects approximately 10% of women of reproductive age worldwide and in the UK. It is as common as diabetes and asthma. Despite this prevalence, it remains significantly underdiagnosed — the average diagnostic delay of 8 years in the UK reflects a combination of symptom normalisation (“it’s just bad periods”), variation in clinical awareness, and the fact that symptoms overlap with many other conditions.

Endometriosis is not confined to women who have had children or who are older — it can develop in adolescence, and symptoms often begin with the first menstrual periods.

What Causes Endometriosis?

The exact cause remains unclear despite decades of research. Several mechanisms are thought to contribute:

• Retrograde menstruation — the most widely accepted theory; menstrual blood flows backwards through the fallopian tubes into the pelvis rather than entirely out of the body. Most women have some retrograde menstruation, but only some develop endometriosis, suggesting additional factors determine whether implants establish and persist.
• Immune system dysfunction — in most women, the immune system clears retrograde menstrual debris. In endometriosis, immune surveillance appears impaired, allowing ectopic tissue to establish and evade clearance.
• Genetic factors — endometriosis has a clear hereditary component; first-degree relatives of women with endometriosis have a 7–10× increased risk.
• Stem cell and metaplasia theories — suggest that cells at ectopic sites differentiate into endometrial-like tissue, rather than arising from transplanted cells.

Symptoms

The symptoms of endometriosis are varied and inconsistent — not all women have all symptoms, and severity correlates poorly with the extent of disease visible at surgery. This is partly why diagnosis is so often delayed.

• Dysmenorrhoea (period pain) — often severe, progressively worsening over successive cycles, not adequately controlled by standard analgesia; typically begins before bleeding and persists throughout
• Chronic pelvic pain — pain present throughout the cycle, not only at menstruation; deep, aching, often worse in certain positions
• Deep dyspareunia — pain during or after sex; characteristically deep pain rather than superficial discomfort; one of the most specific symptoms of endometriosis, particularly in the rectovaginal pouch
• Dysuria and bowel symptoms — pain on urination or defaecation, particularly at the time of menstruation; blood in urine or stool during periods (less common)
• Subfertility — endometriosis is found in an estimated 25–50% of women with unexplained infertility; mechanism involves anatomical distortion, inflammation, impaired implantation
• Fatigue — often profound; a combination of chronic pain, disrupted sleep, and systemic inflammatory effects
• Bloating — cyclical abdominal bloating associated with bowel involvement or pelvic inflammation; sometimes called “endo belly”

Types and Locations

Diagnosis and the Diagnostic Delay

Diagnosis of endometriosis is challenging and the UK diagnostic delay of 8 years reflects real systemic barriers. The only definitive diagnosis is surgical — laparoscopy with histological confirmation of biopsied lesions. Clinical diagnosis based on symptoms and examination is used to guide management while awaiting specialist assessment.

Pelvic ultrasound can identify ovarian endometriomas and some forms of deep disease but cannot exclude endometriosis — a normal ultrasound does not rule it out. MRI is useful for mapping deep infiltrating endometriosis pre-operatively but is not a screening tool.

NICE guidance (NG73) recommends that GPs consider endometriosis in women with cyclical or non-cyclical pain in the lower abdomen, pelvis, or lower back; dyspareunia; cyclical bowel or bladder symptoms; and infertility. The guidance explicitly states that a normal examination or negative ultrasound should not prevent referral for further investigation.

Staging

The American Society for Reproductive Medicine (ASRM) staging system grades endometriosis from I (minimal) to IV (severe) based on the location, extent, and depth of lesions, and the presence of adhesions and endometriomas. Stage IV does not necessarily mean more severe pain — it reflects the anatomical extent of disease, not symptom severity.

Treatment Overview

Endometriosis has no cure, but symptoms are highly manageable. Treatment aims to relieve pain, preserve or restore fertility where desired, and prevent disease progression. Most treatment is hormonal — suppressing the oestrogen-driven growth of ectopic endometrial tissue.

When to Seek Help

• Period pain that is significantly affecting your quality of life or not controlled by OTC pain relief
• Pain during sex
• Cyclical bowel or bladder symptoms — pain or bleeding at menstruation
• Difficulty conceiving after 6–12 months of trying
• Any combination of the above symptoms alongside general fatigue and bloating

If you recognise these symptoms, start with your GP. NICE guidance supports early referral to gynaecology for suspected endometriosis rather than a prolonged trial of empirical treatment.