Endometriosis and Ultrasound: Can a Scan Diagnose Endometriosis?

Endometriosis is a condition in which tissue similar to the endometrium (the lining of the uterus) grows outside the uterus — most commonly on the ovaries, fallopian tubes, peritoneum, bowel, and bladder. It affects approximately 1.5 million women in the UK and is the second most common gynaecological condition after fibroids.

The diagnostic delay for endometriosis in the UK averages 8 years from symptom onset to diagnosis. This delay occurs because symptoms — including dysmenorrhoea (painful periods), dyspareunia (pain during intercourse), chronic pelvic pain, and subfertility — overlap with many other conditions, and because standard pelvic examination and routine ultrasound frequently appear normal in women with superficial peritoneal endometriosis.

A standard pelvic ultrasound scan — as performed in most NHS and private settings — is excellent for identifying ovarian cysts, fibroids, and endometrial pathology. It will detect endometriomas (ovarian cysts containing old blood, characteristic of endometriosis) reliably. However, it will miss superficial peritoneal endometriosis (the most common form) and deep infiltrating endometriosis (DIE) affecting the bowel, bladder, and uterosacral ligaments in the majority of cases.

The key limitation of standard scanning is that it does not include a systematic assessment of the posterior compartment (pouch of Douglas, uterosacral ligaments, rectovaginal septum) or the anterior compartment (bladder, vesicouterine pouch). These areas require a specific examination protocol — the "soft markers" approach — that is only performed by sonographers with specialist endometriosis training.

The specialist endometriosis ultrasound protocol, as described by the British Society of Gynaecological Imaging (BSGI) and the International Deep Endometriosis Analysis (IDEA) group, includes a systematic assessment of the uterus (for adenomyosis), both ovaries (for endometriomas and mobility), the pouch of Douglas (for obliteration), the uterosacral ligaments, the rectosigmoid colon, the bladder, and the ureters.

The "sliding sign" — assessed by gently moving the uterus with the transvaginal probe and observing whether the rectum slides freely over the posterior uterine wall — is a key indicator of posterior compartment adhesions. A negative sliding sign (no movement) strongly suggests obliteration of the pouch of Douglas by endometriosis.

Endometriomas — ovarian cysts containing old menstrual blood — have a highly characteristic ultrasound appearance: a unilocular (single-chambered) cyst with homogeneous low-level internal echoes, often described as "ground glass" or "chocolate" in appearance. This appearance is so distinctive that ultrasound has a sensitivity of approximately 90% and specificity of 94% for endometrioma diagnosis.

"The ground glass appearance of an endometrioma on ultrasound is so characteristic that it can be diagnosed with confidence by an experienced sonographer without the need for surgery."

Endometriomas are associated with reduced ovarian reserve — women with endometriomas have lower antral follicle counts (AFC) and anti-Müllerian hormone (AMH) levels than age-matched controls. For this reason, our Weymouth Street clinic routinely assesses ovarian reserve (AFC) at the same appointment as endometrioma assessment.

Deep infiltrating endometriosis (DIE) is defined as endometriosis that penetrates more than 5 mm beneath the peritoneal surface. It most commonly affects the uterosacral ligaments (69%), rectovaginal septum (14%), bladder (9%), and ureters (3%). DIE is the most symptomatic form of endometriosis and the most challenging to diagnose without specialist imaging.

Bowel endometriosis — affecting the rectosigmoid colon — presents on ultrasound as a hypoechoic (dark) nodule on the anterior wall of the bowel, often with a characteristic "mushroom cap" appearance. Bladder endometriosis appears as a hypoechoic nodule on the posterior bladder wall, typically at the bladder dome or trigone.

Adenomyosis and endometriosis frequently coexist — studies suggest that up to 50% of women with endometriosis also have adenomyosis. The two conditions share symptoms (dysmenorrhoea, heavy periods, pelvic pain) but have distinct ultrasound appearances and different treatment implications.

Adenomyosis is characterised on ultrasound by an enlarged, globular uterus; asymmetric myometrial thickening; myometrial cysts; fan-shaped shadowing; and a poorly defined endometrial-myometrial junction. The MUSA (Morphological Uterus Sonographic Assessment) criteria provide a standardised framework for adenomyosis diagnosis that our HCPC Registered sonographers apply at every specialist pelvic assessment.