An endometrioma is a cyst that forms on the ovary when tissue similar to the uterine lining grows there and bleeds with each menstrual cycle, creating a pocket filled with old, dark blood. This distinctive dark brown fluid is why endometriomas are commonly called “chocolate cysts.” They are one of the most recognizable forms of endometriosis and can range from less than a centimeter to well over 10 centimeters in diameter.
How Endometriomas Form
Your uterine lining sheds each month during your period. In endometriosis, tissue that behaves like this lining ends up outside the uterus, often on or near the ovaries. This displaced tissue responds to the same hormonal signals as normal uterine lining: it thickens, breaks down, and bleeds with every menstrual cycle. But unlike a normal period, the blood has nowhere to go. Over time, it accumulates inside the ovary and forms a cyst.
Researchers have identified two main pathways for how this happens. In the first, endometrial tissue invades a pre-existing functional cyst (the kind that forms naturally during ovulation), essentially converting it into an endometrioma. The cyst may rupture or develop a small perforation that allows the endometrial tissue to move in. In the second pathway, patches of endometriosis on the ovary’s surface bleed directly into the ovarian tissue beneath, creating a new cyst from scratch. This second type tends to be smaller and more densely fibrous. Both pathways likely occur, which explains why endometriomas can look quite different from one person to the next.
Unlike a typical blood-filled cyst that the body gradually reabsorbs, an endometrioma’s interior is lined with sticky endometrial tissue and glands. This lining keeps the cyst biologically active, adding new blood with each cycle and making it progressively harder for the body to clear.
Common Symptoms
Many endometriomas cause no symptoms at all and are discovered incidentally during an ultrasound for something else. When symptoms do appear, they overlap heavily with general endometriosis symptoms, making it difficult to pinpoint the cyst as the specific cause. The most common complaints include pelvic pain (especially during periods or intercourse), heavy or irregular menstrual bleeding, and difficulty getting pregnant.
Some people notice a deep, dull ache on one side of the pelvis that worsens around menstruation. Larger cysts can create a sense of pressure or fullness in the lower abdomen. In rare cases, an endometrioma can rupture, spilling its contents into the pelvic cavity. This causes sudden, sharp pain and can lead to adhesions, bands of scar tissue that stick pelvic organs together. Repeated small ruptures over time are also thought to contribute to the dense adhesions commonly found around endometriomas during surgery.
How Endometriomas Are Diagnosed
Transvaginal ultrasound is the primary tool for identifying endometriomas, and they have a characteristic appearance that experienced sonographers can recognize with high accuracy. On ultrasound, a typical endometrioma shows up as a single-chambered cyst filled with a hazy, evenly distributed pattern called “ground glass” echogenicity. This appearance comes from the thick, old blood swirling inside the cyst. In one large study of over 500 confirmed endometriomas, 84% displayed this ground glass pattern and about 72% were single-chambered cysts.
These features help distinguish endometriomas from other ovarian cysts. A simple fluid-filled cyst appears completely black on ultrasound, while a dermoid cyst (which contains hair, skin cells, or fat) has a much more varied, often brightly echoing interior. Hemorrhagic cysts from ovulation can sometimes mimic the ground glass look, but they typically resolve within one to two menstrual cycles. If a cyst persists, it’s more likely an endometrioma.
When an ovarian mass lacks the classic ground glass pattern, or when it appears in a postmenopausal patient, clinicians exercise extra caution. These are the scenarios where endometriomas are most likely to be confused with other masses, including rare malignancies. MRI can provide additional detail when ultrasound findings are uncertain.
Effects on Fertility and Ovarian Reserve
Endometriomas can reduce fertility in several ways. The cyst itself takes up space within the ovary, potentially displacing or damaging the surrounding pool of eggs. The chronic inflammation associated with endometriosis can also impair egg quality and interfere with ovulation.
One measurable consequence is a decline in ovarian reserve, the number of eggs remaining in the ovaries. Anti-Müllerian hormone (AMH), a blood marker that reflects ovarian reserve, is significantly lower in people with endometriomas compared to those with other types of benign ovarian cysts or healthy ovaries. On average, AMH levels are about 0.84 ng/ml lower in people with endometriomas. What’s more concerning is the rate of decline: AMH drops roughly 26% over six months in people with endometriomas, compared to about 7% in similarly aged healthy individuals. This suggests that endometriomas actively accelerate the loss of ovarian reserve over time, not just at a single point.
This creates a difficult dilemma when it comes to surgery. While removing the cyst eliminates a source of ongoing damage, the surgery itself removes some healthy ovarian tissue along with it. In one study, AMH levels dropped by 48% in the month following surgical excision of an endometrioma. About 80% of surgically removed endometrioma specimens contained ovarian tissue, compared to only 17% for dermoid cyst removals, highlighting how tightly endometriomas embed themselves into the surrounding ovary. This permanent reduction in ovarian reserve is a major reason fertility specialists weigh surgical decisions so carefully.
Treatment Options
Hormonal Therapy
Several hormonal medications can shrink endometriomas without surgery. A large meta-analysis found that a progestin called dienogest reduced cyst diameter by an average of 1.32 cm, while oral contraceptive pills achieved about 1.06 cm of reduction. Other options, including certain injections that temporarily suppress ovarian function, reduced diameter by roughly 1.17 cm. These treatments work by suppressing the hormonal signals that drive monthly bleeding within the cyst, essentially starving it of the fuel that makes it grow.
Hormonal therapy won’t make most endometriomas disappear entirely, but it can bring meaningful size reduction and symptom relief. It’s often the first line of treatment for smaller, stable cysts, and it can also help prevent recurrence after surgery.
Surgery
Surgery is generally considered when an endometrioma causes significant pain, when there’s concern it could be something other than an endometrioma, or when it interferes with fertility treatment (for instance, blocking access to eggs during retrieval). No major medical society has set a firm size cutoff for operating. The European Society of Human Reproduction and Embryology explicitly states that surgery should not be decided based on cyst diameter alone. Some guidelines reference 3 cm as a point where surgery becomes a reasonable consideration, but the decision always depends on the full picture: your symptoms, fertility plans, and whether the cyst appears stable over time.
Laparoscopic cystectomy, where the cyst wall is peeled away from the ovary through small incisions, is the standard surgical approach. It provides the best chance of removing the entire cyst lining and obtaining tissue for examination. However, recurrence rates after surgery range from 30% to 50%, reflecting how difficult it is to eliminate the underlying disease. Simple needle aspiration (draining the cyst with a needle) has recurrence rates above 80% and is not recommended as a standalone treatment.
For people planning pregnancy, the timing of surgery relative to fertility treatment is a critical decision. Given the documented hit to ovarian reserve, many specialists now favor a more conservative approach: monitoring stable cysts closely rather than automatically operating, especially in people with already-reduced ovarian reserve.
Risk of Malignant Transformation
Endometriomas have a small but real risk of transforming into ovarian cancer. This occurs in up to about 1% of people with endometriosis, with the ovary being the most common site. The cancers that develop from endometriomas are typically clear cell or endometrioid subtypes rather than the more common serous type. They tend to be diagnosed at an earlier stage, occur in younger patients, and carry a somewhat better prognosis after surgery than other ovarian cancers.
This low but non-negligible risk is one reason that any endometrioma with atypical ultrasound features, rapid growth, or new solid components warrants further evaluation. In postmenopausal patients, the threshold for concern is even lower, since endometriomas should not be developing or growing after menstruation has stopped.