A chocolate cyst is a type of ovarian cyst filled with old, dark brown blood that resembles melted chocolate. Its medical name is an ovarian endometrioma, and it forms when tissue similar to the uterine lining grows on or inside an ovary. About 10% of people who menstruate have endometriosis, and roughly 17% to 44% of them will develop one or more of these cysts.
How Chocolate Cysts Form
The tissue lining the uterus normally sheds each month during a period. In endometriosis, cells similar to that lining end up in places they don’t belong. The most widely accepted explanation is retrograde menstruation: during a period, some menstrual blood flows backward through the fallopian tubes and into the pelvic cavity. When these displaced cells land on an ovary, they can attach, take hold, and keep responding to hormonal cycles just like normal uterine lining would.
Each month, the misplaced tissue bleeds, but the blood has nowhere to drain. Over time, it pools inside a pocket on the ovary, forming a cyst. The trapped blood darkens and thickens as it ages, turning that characteristic deep brown color. The repeated bleeding also triggers inflammation and scar tissue (adhesions) around the ovary and nearby structures, which contributes to pain and can affect fertility. Fragile, abnormal blood vessels within the cyst wall lead to ongoing micro-bleeding that fuels further growth and inflammation.
Common Symptoms
Pelvic pain or tenderness is the most common symptom, and it can occur at any time, not only during your period. Many people also experience:
- Intensely painful periods
- Pain during sex
- Pain when urinating or having a bowel movement
- Frequent urge to urinate
- Lower back pain
- Bloating, nausea, or vomiting
Some chocolate cysts cause no noticeable symptoms at all and are discovered incidentally during an ultrasound for something else. The severity of symptoms doesn’t always match the size of the cyst. A small endometrioma surrounded by dense adhesions can be more painful than a larger one that sits relatively freely on the ovary.
How They’re Diagnosed
Transvaginal ultrasound is the primary tool for identifying a chocolate cyst. On the screen, the old blood inside the cyst creates a hazy, uniform pattern that radiologists call “ground glass” echogenicity. A typical endometrioma has one to four compartments, this ground-glass appearance, and no solid growths with their own blood supply inside the cyst wall.
That last detail matters. Using color Doppler (a mode that highlights blood flow), doctors can distinguish a chocolate cyst from something more concerning. Solid-looking clumps of fibrin or old blood clots inside an endometrioma won’t show blood flow, while true solid tissue in a cancerous cyst will. When cysts look atypical on ultrasound, this difference in blood flow is a key way to tell them apart.
Effects on Fertility
Chocolate cysts can impair fertility through several routes. The chronic inflammation and scar tissue they cause can distort the ovary and block or damage the fallopian tubes. The cyst itself may also harm healthy ovarian tissue over time, reducing the number of eggs the ovary can produce.
Surgery to remove the cyst (cystectomy) improves symptoms and can help with fertility, but it comes with a trade-off. A meta-analysis of surgical outcomes found that a key marker of ovarian reserve dropped by an average of 35% to 54% in the months following cystectomy. This decline was consistently measurable at every follow-up point, from a few weeks after surgery to more than a year later. For anyone considering future pregnancy, this is an important factor in deciding whether and when to operate.
Treatment Options
There is no single size cutoff that automatically triggers surgery. Current European and UK guidelines recommend basing the decision on the full picture: your symptoms, whether there’s any concern about malignancy, your fertility plans, and whether the cyst is blocking access to eggs for assisted reproduction.
Hormonal treatments, such as birth control pills or medications that temporarily suppress estrogen, can slow cyst growth and manage pain. One class of drugs (GnRH agonists) has been shown to shrink endometriomas, though it doesn’t reliably improve pain on its own. These medications work by quieting the hormonal cycle that fuels the misplaced tissue, but cysts typically return once the medication stops.
When surgery is chosen, laparoscopic cystectomy (removing the cyst wall through small incisions) is the standard approach. It offers good symptom relief and allows the tissue to be examined under a microscope. However, recurrence is a real possibility. In a study following 224 patients for at least two years after laparoscopic excision, 30.4% developed a new endometrioma. Many doctors recommend hormonal therapy after surgery to lower that recurrence risk.
Rupture Risk
Like other ovarian cysts, a chocolate cyst can rupture. When it does, the thick, dark blood spills into the pelvic cavity, causing sudden, sharp pain in the lower abdomen or back, vaginal spotting, and bloating. A rupture paired with severe nausea and vomiting could signal ovarian torsion (the ovary twisting on itself), which requires emergency treatment. Fever, heavy vaginal bleeding, or faintness after sudden pelvic pain are also signs that warrant immediate medical attention.
Connection to Ovarian Cancer
The overall risk is low but not zero. A large prospective study tracking 6,398 women with endometriomas over a median of nearly 13 years found that 46 (0.72%) were later diagnosed with ovarian cancer. The specific cancer types linked to endometriosis are clear cell, endometrioid, and serous ovarian cancers. This is one reason doctors monitor endometriomas with periodic ultrasounds, especially if the cyst changes in appearance or develops new features like solid areas with blood flow.