What Is an Endometrioma? Ovarian Endometriosis Explained

What is endometriosis?

Endometriosis is a condition in which tissue similar to the lining of the uterus is found outside the uterine cavity.

It commonly affects structures within the pelvis.

There are several recognised forms, including:

• —superficial peritoneal endometriosis
• —ovarian endometriosis/endometriomas
• —deep endometriosis.

A person can have more than one type at the same time.

How does an endometrioma develop?

When endometriosis affects the ovary, repeated cyclical bleeding and inflammation can contribute to formation of a cyst filled with altered blood products.

Over time, the blood becomes concentrated and degraded.

This creates the characteristic appearances sometimes seen on ultrasound and MRI.

What symptoms can an endometrioma cause?

Some people have no symptoms and the cyst is found incidentally.

Others may experience symptoms associated with endometriosis, including:

• —painful periods
• —pelvic pain
• —pain during or after sex
• —pain around ovulation
• —fertility difficulties
• —bowel or urinary symptoms when other endometriosis is present.

The amount of pain does not necessarily correlate with the size of the endometrioma.

A small endometrioma can occur in somebody with substantial symptoms, while a larger one may occasionally be discovered unexpectedly.

How is an endometrioma diagnosed?

Imaging is extremely useful.

Current NICE guidance recommends transvaginal ultrasound for people with suspected endometriosis, including when pelvic examination is normal.^[1]

• —ovarian endometriomas
• —deep endometriosis
• —other ovarian or pelvic abnormalities.

MRI may be useful in selected cases, particularly where:

• —the ultrasound appearance is uncertain
• —deep endometriosis is suspected
• —disease mapping is required
• —surgery is being planned
• —the pelvic anatomy is complex.

What does an endometrioma look like on ultrasound?

A typical endometrioma often appears as an ovarian cyst containing relatively homogeneous low-level internal echoes.

This is sometimes described as a “ground-glass” appearance.

Other supportive features may be present.

However:

Not every endometrioma looks textbook, and not every cyst with internal echoes is an endometrioma.

Radiologists and sonographers consider the complete appearance of the ovary and pelvis.

A 2024 systematic review and meta-analysis found high diagnostic accuracy for transvaginal ultrasound in detecting endometriomas.^[2]

What does an endometrioma look like on MRI?

Blood products have characteristic MRI appearances.

Endometriomas often demonstrate:

• —High signal on T1-weighted imaging — Blood products within the cyst can appear bright on T1-weighted sequences.
• —Persistence of T1 signal after fat suppression — This helps distinguish blood products from fat-containing lesions.
• —T2 shading — The cyst may become progressively darker on T2-weighted imaging because of concentrated blood products and repeated haemorrhage. This classic appearance is known as the T2 shading sign.

MRI can also assess whether there is associated deep endometriosis elsewhere in the pelvis.

Is ultrasound or MRI better?

For identifying a typical ovarian endometrioma, both specialist transvaginal ultrasound and MRI can perform very well.

A 2024 systematic review and meta-analysis involving 1,976 participants found high diagnostic accuracy for both modalities, without evidence of a significant difference between them in the available comparative data.^[2]

Ultrasound is usually more accessible and forms part of the first-line pathway.

MRI becomes particularly useful when broader pelvic mapping or further characterisation is required.

The quality and expertise of the examination also matter.^[3]

Can a normal ultrasound rule out endometriosis?

No.

Even if no endometrioma or deep endometriosis is identified, superficial endometriosis may still be present.

Current NICE guidance explicitly states that endometriosis should not be excluded simply because examination and ultrasound are normal.^[1]

Can an endometrioma affect fertility?

Endometriosis and endometriomas can be associated with reduced fertility.

The relationship is complex.

Factors may include:

• —ovarian inflammation
• —the underlying endometriosis
• —ovarian reserve
• —adhesions
• —tubal factors
• —age
• —other fertility factors.

Having an endometrioma does not mean that pregnancy is impossible.

Many people with endometriomas conceive naturally or with fertility treatment.

What is ovarian reserve?

Ovarian reserve describes the remaining pool of follicles within the ovaries.

It can be estimated using measures such as:

• —anti-Müllerian hormone — AMH
• —antral follicle count on ultrasound.

These tests do not provide a perfect prediction of an individual's fertility, but they can help clinicians understand ovarian response in some fertility contexts.

Can surgery affect ovarian reserve?

Yes.

This is an important part of decision-making.

Removing an endometrioma can potentially remove or damage some normal ovarian tissue and affect ovarian blood supply.

Studies have demonstrated reductions in markers such as AMH after some forms of endometrioma surgery.^[4,5]

This does not mean surgery should never be performed.

Instead, it means the potential benefits and risks need to be weighed carefully, particularly when:

• —both ovaries are affected
• —somebody has had previous ovarian surgery
• —ovarian reserve is already reduced
• —future fertility is important.

Does every endometrioma need to be removed?

No.

The decision is individual.

Possible considerations include:

• —severity of symptoms
• —cyst size
• —cyst appearance
• —change over time
• —fertility plans
• —age
• —ovarian reserve
• —previous surgery
• —whether malignancy is a concern
• —whether other deep endometriosis requires treatment.

Some people are managed without surgery.

Others may benefit from surgery.

What does NICE recommend?

NICE recommends referral to a specialist endometriosis service for suspected or confirmed ovarian endometrioma.^[1]

For people who choose not to undergo surgery, NICE advises considering outpatient follow-up, with or without examination and pelvic imaging, particularly where there is:

• —one or more endometrioma larger than 3 cm.^[1]

This does not mean that every cyst above 3 cm must be operated on.

It means that ongoing specialist assessment may be appropriate.

What surgery is performed?

One surgical approach is laparoscopic ovarian cystectomy, where the cyst wall is removed while attempting to preserve healthy ovarian tissue.

Other techniques involve ablation of the inner cyst lining.

There are trade-offs.

Cystectomy may reduce recurrence compared with some drainage/ablation approaches, while surgery can also affect ovarian reserve.

The European Society of Human Reproduction and Embryology therefore emphasises individualised decision-making.^[4]

What about aspiration or simply draining the cyst?

Simply draining an endometrioma without addressing the cyst lining is generally associated with a higher recurrence risk.

Management should be planned by an appropriately experienced gynaecology/endometriosis team rather than treating it like an ordinary simple ovarian cyst.

Does endometrioma surgery improve fertility?

This is not a simple yes-or-no question.

Surgery may be appropriate in particular circumstances, but operating on an endometrioma before fertility treatment does not automatically improve the chance of pregnancy and may reduce ovarian reserve.

Fertility priorities should therefore be discussed before surgery where possible.

Depending on the individual, discussion may involve:

• —gynaecology
• —specialist endometriosis services
• —fertility specialists.

What if I am having IVF?

The presence of an endometrioma does not automatically mean it should be removed before assisted reproduction.

Potential reasons for surgery need to be balanced against possible loss of ovarian reserve.

This decision should be individualised with the fertility and endometriosis teams.

Can endometriomas occur in both ovaries?

Yes.

Endometriomas can be:

• —unilateral — affecting one ovary
• —bilateral — affecting both ovaries.

Bilateral disease is especially relevant when considering ovarian reserve and surgery.

Can an endometrioma come back after surgery?

Yes.

Recurrence can occur.

The likelihood varies according to factors including:

• —surgical technique
• —underlying disease
• —duration of follow-up
• —postoperative treatment
• —individual characteristics.

Surgery does not necessarily “cure” the underlying tendency to develop endometriosis.

Does an endometrioma mean I have severe endometriosis?

Not necessarily in terms of symptoms.

An endometrioma confirms ovarian involvement, and additional endometriosis elsewhere in the pelvis may be present.

However, the amount of visible disease does not reliably predict how much pain somebody experiences.

NICE recommends basing treatment decisions on the person's symptoms, preferences and priorities rather than stage alone.^[1]

Can an endometrioma become cancerous?

Most endometriomas are benign.

Certain types of ovarian cancer have an association with endometriosis, but malignant transformation of an individual endometrioma is uncommon.

Imaging features that are unusual for a typical endometrioma may lead the radiologist or gynaecologist to recommend additional investigation.

Do not interpret this association as meaning:

“An endometrioma will turn into cancer.”

That is not the usual course.

Why might my scan need specialist review?

Endometriosis imaging is highly dependent on both technique and experience.

Dedicated ultrasound and MRI protocols can perform better than generic pelvic examinations when deep endometriosis is being assessed.^[3]

An endometrioma may also be a clue that there is disease elsewhere.

The radiologist or sonographer may therefore assess:

• —ovaries
• —uterosacral ligaments
• —posterior pelvis
• —bowel
• —bladder
• —ureters
• —pelvic sidewalls
• —evidence of adhesions or distorted anatomy.

What questions should I ask after finding out I have an endometrioma?

Useful questions include:

• —How large is it?
• —Is it on one ovary or both?
• —Does it look typical for an endometrioma?
• —Is there evidence of deep endometriosis elsewhere?
• —Do I need specialist referral?
• —Should the cyst be monitored?
• —What are my options for symptom control?
• —Would surgery benefit me?
• —Could surgery affect my ovarian reserve?
• —Should fertility be discussed before surgery?

When should I seek urgent medical attention?

Most endometriomas are managed routinely.

However, sudden severe pelvic or abdominal pain — especially with:

• —vomiting
• —fainting
• —significant bleeding
• —fever
• —feeling acutely unwell

requires urgent medical assessment because several acute gynaecological conditions can cause these symptoms.

Do not assume severe new pain is simply “my endometriosis”.

The key message

An endometrioma is ovarian endometriosis — but the scan finding alone does not determine the treatment.

The right approach depends on:

• —your symptoms + the imaging + your fertility plans + ovarian reserve + your priorities.

For some people, monitoring and medical treatment are appropriate.

For others, surgery may be beneficial.

The decision should be individualised.