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.

Recent data from Endometriosis UK shows the average diagnostic delay has increased to 9 years and 4 months — up from 8 years in 2020.[reference:0]Shockingly, 39% of those surveyed needed to visit their GP 10 times or more before endometriosis was suspected, and 55% attended A&E with their symptoms — with 46% of those sent home without treatment.[reference:1]

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. NICE now explicitly requires that young women or people under 17 should be referred to specialist adolescent services, ensuring age-appropriate care.[reference:2]

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. NICE now emphasises the relevance of family history in diagnosis, which should lead to earlier consideration of endometriosis in young women with affected relatives. A positive history in a first-degree relative increases the likelihood of developing endometriosis.[reference:3]
• 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

Adenomyosis: The Close Relative

Adenomyosis is a closely related condition in which endometrial tissue grows into the muscular wall of the uterus (myometrium) rather than outside it. It causes the uterus to become enlarged, thickened, and tender. It is an important differential diagnosis for endometriosis, and the two conditions frequently co-exist.

A key study comparing symptom profiles found that heavy menstrual bleeding was significantly more frequent in women with adenomyosis (64%) than in those with endometriosis (19%). However, endometriosis was linked to more severe pain-related symptoms — including dysmenorrhoea, severe dysmenorrhoea, and chronic pelvic pain. Dyspareunia was common in both conditions.[reference:4]

Women with both conditions experience the highest overall symptom burden. This distinction matters clinically because it guides the choice of treatment — adenomyosis responds differently to hormonal therapies, and the levonorgestrel IUS (Mirena) is particularly effective for adenomyosis-related heavy bleeding.

Diagnosis and the Diagnostic Delay

Diagnosis of endometriosis is challenging and the UK diagnostic delay of 8–9 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. NICE guidance states explicitly: Do not exclude the possibility of endometriosis if transvaginal ultrasound scan is normal and history is suggestive.[reference:5]

MRI is useful for mapping deep infiltrating endometriosis pre-operatively but is not a screening tool. Serum CA125 should not be used to diagnose endometriosis.[reference:6]

NICE guidance (NG73, updated 2024/2025) 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. Key updates in the 2025 guidance include:

• Women should be offered transvaginal ultrasound referral even when physical examination results are normal
• Women should be offered abdominal examination if pelvic examination is unsuitable or declined
• Young women under 17 should be referred to specialist adolescent services
• GPs can simultaneously treat endometriosis whilst waiting for further investigations
• People should be offered laparoscopy even if ultrasound and MRI results are normal
• Laparoscopic drainage and ablation of endometriomas can be offered as an alternative to cystectomy, helping to preserve reproductive function

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. The scoring system is:

Endometriosis and Fertility

Endometriosis is found in an estimated 25–50% of women with unexplained infertility. The mechanisms by which endometriosis affects fertility are multifactorial:

• Anatomical distortion — adhesions and endometriomas can distort the fallopian tubes and ovaries, impairing egg release and transport
• Inflammation — chronic pelvic inflammation creates an environment hostile to sperm, egg, and embryo
• Impaired implantation — inflammatory mediators can affect endometrial receptivity
• Reduced oocyte quality — particularly in advanced stages (III–IV), endometriosis is associated with reduced oocyte quality and lower ovarian reserve

The impact on IVF outcomes varies by stage. For stage I–II endometriosis, IVF live birth rates are comparable to those without endometriosis. For stage III–IV disease, live birth rates, clinical pregnancy rates, and mean number of oocytes retrieved are significantly lower compared to no endometriosis. Ovarian endometriomas and previous ovarian surgery are most consistently associated with reduced ovarian reserve and lower oocyte yield.[reference:7][reference:8]

However, many women with endometriosis conceive naturally, and IVF remains a highly effective option. NICE published a dedicated fertility pathway for endometriosis in 2026, reflecting the importance of specialist fertility assessment for those struggling to conceive.[reference:9]

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.

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.