Endometriomas are fluid-filled cysts that form on the ovaries as a result of endometriosis. They contain thick, dark brown blood and tissue, which is why they’re often called “chocolate cysts.” These cysts develop in roughly 17 to 44% of people with endometriosis and make up about 35% of all benign ovarian cysts.
How Endometriomas Form
Endometriomas develop when tissue similar to the uterine lining grows on or inside an ovary. This misplaced tissue responds to your hormonal cycle just like the lining of your uterus does: it thickens, breaks down, and bleeds with each menstrual period. But because the blood has no way to exit, it accumulates inside a cyst on the ovary. Over time, the trapped blood thickens and darkens, giving the cyst its characteristic chocolate-brown appearance.
Unlike a normal blood-filled pocket that the body might reabsorb, endometriomas are lined with sticky tissue containing glands and extra fibrous material. This lining keeps the cyst intact and makes it more likely to persist or grow. The tissue also releases inflammatory molecules that contribute to pain and can damage the surrounding ovary.
The most widely accepted explanation for how endometrial tissue reaches the ovary is retrograde menstruation, where menstrual blood flows backward through the fallopian tubes into the pelvic cavity, carrying endometrial cells that then implant on the ovary. Other theories suggest that cells already present in the pelvic lining can transform into endometrial-like tissue under the influence of hormones or immune system changes. A newer theory points to stem cells that may travel from the uterus and establish endometriotic growths with an unusual ability to regenerate.
Common Symptoms
Many endometriomas cause no symptoms at all and are found incidentally during an ultrasound for something else. When symptoms do appear, they overlap heavily with endometriosis in general. The hallmark is pelvic pain that goes beyond normal menstrual cramping. If your period pain is severe enough to keep you home from work or school, that’s not typical cramping.
Other common symptoms include:
- Cramping that starts before your period and continues after bleeding stops
- Lower back or abdominal pain that may be constant or cyclical
- Pain during sex
- Pain with bowel movements or urination
- Fatigue, bloating, constipation, or nausea, particularly around your period
- Difficulty getting pregnant
The size of the cyst doesn’t always predict how much pain you’ll feel. A small endometrioma can cause significant discomfort if it’s triggering inflammation in surrounding tissue, while a larger one might be painless.
How They’re Diagnosed
Transvaginal ultrasound is the primary tool for identifying endometriomas, and experienced sonographers can spot them with good accuracy. The classic appearance on ultrasound is a single-chambered cyst filled with a hazy, low-level pattern called “ground glass” echogenicity. In a large study of confirmed endometriomas, about 84% displayed this ground glass pattern and 72% were single-chambered. The median size at diagnosis was about 5 centimeters.
When a cyst doesn’t show that typical ground glass appearance, or when it’s found in someone who has already gone through menopause, clinicians take extra care to rule out other possibilities, including ovarian cancer. Additional imaging or blood tests may be used in those cases. A definitive diagnosis requires examining the tissue under a microscope after surgical removal, but most endometriomas are confidently identified on ultrasound alone.
Effects on Fertility
Endometriomas can reduce your ovarian reserve, the pool of eggs available in your ovaries. Women with endometriomas have measurably lower levels of anti-Müllerian hormone (AMH), a blood marker that reflects how many eggs remain. On average, AMH levels are about 0.84 ng/mL lower in women with endometriomas compared to women with other types of benign ovarian cysts or healthy ovaries. The decline also appears to happen faster: one study found AMH dropped by about 26% over time in women with endometriomas, compared to roughly 7% in women without them.
This creates a difficult clinical dilemma because surgery to remove the cyst also damages the ovary. After surgical excision of an endometrioma on one side, AMH levels drop by about 30%. After removal from both ovaries, the decline reaches roughly 44%. In the first month after surgery, AMH can fall by nearly half, and while it partially recovers over the following months, it typically doesn’t return to pre-surgery levels. Surgically treated ovaries also produce fewer eggs during fertility treatments, with about 2 to 3 fewer eggs retrieved from the operated ovary compared to the untreated side.
If you’re planning to pursue pregnancy, either naturally or through fertility treatment, the decision about whether and when to operate involves weighing the cyst’s impact against the surgical cost to your egg supply. An untreated endometrioma also makes egg retrieval harder during IVF, increasing the risk of not being able to access all available follicles by more than three-fold.
Treatment Options
Treatment depends on your symptoms, the size of the cyst, and whether you’re trying to conceive.
Hormonal Management
Hormonal treatments are considered the first line for managing endometrioma symptoms. The goal is to suppress ovulation and stop menstrual cycling, which starves the cyst of the hormonal stimulation it needs to grow. Options include birth control pills taken continuously, progestins, and medications that temporarily shut down ovarian hormone production. These approaches are equally effective at reducing pain, though they differ in side effects. Hormonal therapy is also used after surgery to help prevent the cyst from coming back. It’s important to know that hormonal treatment doesn’t improve fertility and is not recommended for that purpose alone.
Surgical Removal
The general consensus is that endometriomas larger than 4 centimeters should be surgically removed. Surgery at that threshold helps reduce pain, improves the chances of natural conception, confirms the diagnosis through tissue analysis, and rules out anything more serious. The standard approach is laparoscopic cystectomy, a minimally invasive procedure where the cyst wall is separated from the ovary and removed. This produces better outcomes than simply draining the cyst, which has a much higher recurrence rate.
Even with complete excision, endometriomas come back at notable rates. Pooled data from 23 studies found recurrence rates of about 4% at 3 months, 14% at 6 months, 17% at one year, and 27% at two years. Using hormonal therapy after surgery can help lower the odds of recurrence.
Risk of Cancer
Endometriomas are benign, and the vast majority remain so. The risk of malignant transformation is estimated at 0.5 to 1.6%, with most cases developing into clear cell or endometrioid ovarian cancers. This risk is one reason clinicians recommend removing larger cysts and monitoring smaller ones with periodic imaging. Endometriomas found after menopause, or those with unusual features on ultrasound, warrant closer evaluation since the typical hormonal pattern that sustains a benign endometrioma is no longer present.