Endometrioma (Chocolate Cyst): Symptoms, Scans and Treatment

An endometrioma is a cyst that forms on or inside an ovary because of endometriosis. It is filled with thick, dark brown fluid made of old blood, so it is also called a "chocolate cyst".

Endometriomas are common in endometriosis. Between 17 and 44 in 100 people with endometriosis get one. Most are not dangerous, but they can cause pain, they can affect fertility, and the choice about surgery needs care.

This guide covers the symptoms, how an endometrioma shows on a scan, what it means for your fertility, the treatment choices and the warning signs that need urgent care. It is general information and not medical advice.

What is an endometrioma?

In endometriosis, tissue similar to the lining of the womb grows outside the womb. When this tissue grows on an ovary, it can bleed with each cycle. The blood cannot get out, so it collects and makes a cyst. Over time the blood gets old, thick and brown.

There are three main types of endometriosis: tissue on the lining of the pelvis, deep endometriosis that grows into organs, and endometrioma on the ovary. You can have more than one type at the same time. An endometrioma can be on one ovary or on both.

An endometrioma is different from the most common ovarian cyst. Functional cysts form as part of the normal cycle, are usually harmless and often go away in a few months. An endometrioma comes from endometriosis, and it can come back after treatment.

Endometrioma symptoms

The most common symptom is pain or tenderness in the pelvis that can happen at any time, not only during your period. Other symptoms can include:

Very painful periods
Pain during or after sex
Pain when you pee or poo
Needing to pee more often
Lower back pain
Bloating, nausea or vomiting
Difficulty getting pregnant

Some endometriomas cause no symptoms. A small cyst is sometimes found by chance on a scan for something else. A small cyst that causes no symptoms may not need treatment.

How an endometrioma shows on ultrasound

The first test is usually a transvaginal ultrasound, with a small probe put into the vagina.

On ultrasound, the old blood in an endometrioma gives the fluid an even, cloudy look called "ground glass". In a large study of 713 endometriomas, 51% were cysts with one space and ground glass fluid. The typical endometrioma in a person who has not reached menopause has ground glass fluid, one to four spaces inside, and no solid parts.

Endometrioma vs other ovarian cysts on ultrasound

Feature	Endometrioma	Functional cyst
Cause	Endometriosis	Normal cycle
Fluid	Ground glass	Ground glass is less common
Inside	1 to 4 spaces, no solid parts	Varies
Course	Can come back after treatment	Often goes in a few months

The ground glass look and the one to four spaces with no solid parts are found less often in other cysts. Some cysts still look alike on one scan. If the result is not clear, your doctor can ask for a repeat ultrasound in a few weeks. A functional cyst often goes away in a few months, so a change between scans helps to tell them apart.

After menopause, endometriomas are rare. A cyst with ground glass fluid in a person after menopause has a higher risk of being cancer, so it needs more tests.

Endometrioma on MRI

Your doctor may ask for an MRI scan if the ultrasound is not clear or to plan surgery. On MRI, an endometrioma is bright on T1 images and darker on T2 images. Radiologists call this dark look on T2 "shading". It comes from old, thick blood.

MRI can also show deep endometriosis near the ovary, such as on the bowel or the ligaments behind the womb. This helps the surgeon to plan. Remember that a normal ultrasound or MRI does not rule out endometriosis, because small areas on the lining of the pelvis often do not show on a scan. If you still have symptoms, read our guide on how to get diagnosed with endometriosis.

What an endometrioma says about endometriosis stage

An endometrioma may be a sign of moderate to severe endometriosis. Moderate and severe endometriosis are stages 3 and 4 in the most used staging system, so a person with an endometrioma is often given one of these stages.

The stage describes what the surgeon sees in the pelvis. Ask your doctor what your stage means for your treatment and your fertility.

Endometrioma and fertility

You can get pregnant with an endometrioma. It may be more difficult if the cyst is large or if both ovaries have one, because endometriosis can cause scar tissue and inflammation around the ovaries and tubes.

Ovarian reserve and AMH

Ovarian reserve is the number of eggs left in your ovaries. A blood test for anti-Müllerian hormone (AMH) gives an estimate of it. A lower AMH means fewer eggs remain.

Surgery to remove an endometrioma can also remove some healthy ovarian tissue with the cyst wall. In one review of 237 patients, AMH fell after cyst removal by an average of 1.13 ng/ml. This is why surgeons try to protect as much of the ovary as they can, and why it is good to ask for an AMH test before surgery if you want children in the future.

If you plan IVF, surgery is not routinely recommended before IVF only to improve the chance of a baby. It shows no benefit for that, and it is likely to lower your ovarian reserve. Surgery can still help if the pain is bad or if the cyst blocks the way to collect eggs. If there is a lot of endometriosis on the ovaries, your doctor should discuss the pros and cons of egg freezing with you. Its true benefit in endometriosis is not yet known.

Endometrioma treatment

The best choice depends on your symptoms, the size of the cyst, your age, past surgery and whether you want to get pregnant. There are three main choices.

Write down your questions and your symptoms before you talk about treatment.

Watchful waiting

If the cyst is small and causes few or no symptoms, your doctor can check it with repeat scans. You get treatment only if it grows or the pain gets worse.

Hormonal treatment

Hormonal medicines such as hormonal birth control can ease the pain of endometriosis. After surgery, people who do not want to get pregnant soon are offered long-term hormone treatment to help stop the cyst and the pain coming back. For more on these medicines, see our guide to endometriosis treatment options.

Surgery: cystectomy or drainage

Surgery is often offered for cysts that are painful or larger than 4 cm. It is usually done by laparoscopy (keyhole surgery). There are two main ways to treat the cyst:

Method	What the surgeon does	Main point
Cystectomy	Peels out the whole cyst wall	Less chance it comes back
Drainage and burning	Empties the cyst and burns the inside	Comes back more often

Guidelines recommend cystectomy instead of drainage and burning, because it lowers the chance that the cyst and the pain come back. A cyst that is only drained often fills up again. But cystectomy can lower your ovarian reserve, so the surgeon must take care to protect the ovary. The decision should also take into account your age, past surgery, ovarian reserve and wishes.

The surgeon usually treats other endometriosis in the pelvis at the same time. Read about excision versus ablation and how to prepare for laparoscopy and recover.

Questions to ask your surgeon

How large is the cyst, and is it on one ovary or both?
Do you plan cystectomy or drainage, and why?
What is my AMH, and how could surgery change it?
Should I think about egg freezing first?
Will you treat other endometriosis at the same time?
What hormone treatment do you suggest after surgery?

Rupture and torsion: when to get urgent care

Two emergencies are rare but serious. An endometrioma can split open (rupture) and leak old blood into the pelvis. An ovary with a cyst can also twist and block its own blood supply. This is called torsion. Both are emergencies.

Get emergency care at once if you have:

Sudden, severe pain in your tummy that comes without warning or sudden, severe pelvic pain
Pain with fever, nausea or vomiting
Feeling weak, dizzy or faint

If you are not sure, it is always better to get checked. Tell the staff that you have a known endometrioma.

Can an endometrioma turn into cancer?

It is possible but rare. A change to cancer may happen in 1 to 2 in 100 endometriomas. Endometriosis does raise the risk of some cancers a little, but the increase in absolute risk is low compared with people without endometriosis.

On a scan, doctors look for solid parts inside the cyst or ground glass fluid after menopause. These signs lead to more tests. They do not mean you have cancer. Ask your doctor how often you need a scan.

What to track before a scan or surgeon visit

A clear record helps your doctor to compare scans and to decide if treatment helps. Write down:

Pain. How bad it is from 0 to 10, which side, and the days it happens.
Period days. The first day of each period and how heavy the flow is.
Pain with sex, peeing or pooing. Which day and how bad.
Bowel and bloating changes. Constipation, diarrhoea or a swollen belly.
Medicines. What you took, how often and if it helped.
Sudden pain. Any attack of sudden, sharp pain, even if it went away.
Past scans and surgery. Dates, cyst size and the type of surgery.

Endolog is a pain and symptom diary for endometriosis. You log pain severity, where it hurts, bleeding, digestive, bowel and urinary symptoms, fatigue and flare notes on the days they happen. You then have a record to show at your scan or surgeon visit. For a paper version, print the free pain diary.

Frequently asked questions

What is a chocolate cyst?

A chocolate cyst is another name for an endometrioma. It is an ovarian cyst filled with thick, brown fluid made of old blood from endometriosis.

How common is an endometrioma in endometriosis?

Between 17 and 44 in 100 people with endometriosis get one.

How is an endometrioma different from a normal ovarian cyst?

An endometrioma comes from endometriosis and has ground glass fluid on ultrasound. A functional cyst forms in the normal cycle and often goes away in a few months, so a repeat scan can help to tell them apart.

What size endometrioma needs surgery?

Surgery is often offered for cysts larger than 4 cm or cysts that cause pain. The decision also depends on your age, fertility plans and past surgery.

Does an endometrioma always mean stage 4 endometriosis?

No. It may be a sign of moderate to severe endometriosis, often stage 3 or 4, but the stage depends on everything the surgeon sees.

Can an endometrioma come back after surgery?

Yes. The risk depends on the size of the cyst, your age and the type of treatment. Cystectomy and hormone treatment after surgery lower the chance that it comes back.

This article is for general information. It is not medical advice and does not replace advice from your own doctor, who decides what treatment is right for you. If you think you have an emergency, call 911.