Most ovarian cysts appear on ultrasound as small, dark, fluid-filled circles with thin, smooth walls. They look like a black bubble on the screen because the fluid inside doesn’t bounce back sound waves. This is what doctors call a “simple cyst,” and it’s the most common type. But not all cysts look the same. The appearance varies dramatically depending on the type of cyst, what’s inside it, and whether it’s causing problems.
Simple Cysts on Ultrasound
A simple ovarian cyst is the most straightforward to identify. On an ultrasound image, it shows up as a round or oval black space with a bright white outline. The black interior means the cyst is filled with clear fluid. The wall is thin and smooth, with no bumps, ridges, or internal structures. There’s just one single chamber (no dividing walls inside), and no blood flow running through it.
Behind the cyst, you’ll often see a bright band on the ultrasound. This happens because the fluid inside the cyst transmits sound waves easily, making the tissue behind it appear brighter than surrounding areas. Radiologists look for this feature as confirmation that the structure is fluid-filled rather than solid.
How Different Cyst Types Look
The type of cyst determines what you see on the screen and, if surgery happens, what a surgeon finds inside.
Follicular Cysts
These are the most common ovarian cysts and form when an egg follicle doesn’t release its egg during ovulation. On ultrasound, they look like the textbook simple cyst: clear fluid, a very thin wall, and no internal structures. They’re usually small and resolve on their own within one to three menstrual cycles.
Corpus Luteum Cysts
After an egg is released, the follicle normally shrinks. Sometimes it fills with fluid or blood instead. On ultrasound, a corpus luteum cyst can look more complex than a follicular cyst because it may contain fresh blood, which shows up as grey or speckled areas inside the otherwise dark circle. If the cyst bleeds significantly, the blood inside gives it a distinctive swirling or layered appearance. During surgery, the fluid inside functional cysts like these is typically yellow.
Endometriomas (Chocolate Cysts)
Endometriomas form when tissue similar to the uterine lining grows on the ovary. These cysts are filled with old, thickened blood that has a dark brown, chocolate-like consistency. On ultrasound, they have a characteristic “ground-glass” look: instead of appearing completely black like a simple cyst, the interior looks hazy or uniformly grey, almost like frosted glass. This uniform haziness is a strong clue that old blood is pooled inside. During surgery, the dark brown fluid is immediately recognizable.
Dermoid Cysts
Dermoid cysts are among the most visually unusual structures found anywhere in the body. They develop from germ cells (the same cells that form embryos) and can contain fully mature tissue: skin, hair, teeth, sweat glands, nerve tissue, and even small amounts of brain tissue. On ultrasound, they look chaotic. You may see bright white spots (from teeth or bone), mixed grey areas (from fat and sebum), and sometimes the shadowy lines of hair strands floating in oily fluid. The mix of solid and liquid materials inside gives them a distinctive, jumbled appearance that’s quite different from any other cyst type.
What Makes a Cyst Look Concerning
Radiologists evaluate specific visual features to determine whether a cyst is likely benign or potentially malignant. The International Ovarian Tumor Analysis (IOTA) system lays out five benign features and five features associated with malignancy, and the contrast between these two lists gives a clear picture of what “reassuring” versus “worrying” looks like on a scan.
Features that suggest a cyst is benign:
- Single chamber, any size. A cyst with one fluid-filled compartment and no internal walls is reassuring.
- Solid portions smaller than 7 mm. Tiny bits of solid tissue are common in benign cysts.
- No internal blood flow. A cyst with no active blood supply to its walls or contents is less concerning.
- Smooth surface on a multi-chambered cyst under 10 cm. Even cysts with dividing walls can be benign if everything looks smooth and orderly.
Features that raise concern:
- Irregular solid mass. Lumpy, uneven solid tissue inside or around the cyst.
- Four or more finger-like projections. Small bumps or papillary structures growing from the cyst wall into its interior.
- Strong blood flow through solid areas. Active blood supply feeding solid components suggests rapid growth.
- A multi-chambered mass 10 cm or larger with irregular solid areas.
- Free fluid in the abdomen (ascites). Fluid pooling outside the cyst, in the surrounding abdominal cavity.
Wall thickness also matters. Cysts with thin, smooth walls are more often benign. Thick, irregular walls with uneven edges are more often associated with malignancy. Internal dividing walls (septations) thicker than 3 mm, especially those with their own blood supply, also increase suspicion.
What a Surgeon Sees During Surgery
The ultrasound image is a black-and-white interpretation of what’s happening inside the ovary. During laparoscopic surgery, a surgeon sees the cyst in full color, and the visual differences between cyst types become even more distinct.
Functional cysts (follicular and corpus luteum) tend to have few blood vessels on the ovarian surface, and when opened, release yellow fluid. Neoplastic cysts, which are new growths rather than byproducts of the menstrual cycle, have noticeably more blood vessels running across the surface. Their contents vary widely: clear fluid, dark brown old blood, thick mucus, or the oily mix of hair and sebum found in dermoid cysts.
How Large Cysts Change Your Appearance
Most ovarian cysts are small enough that you’d never know they were there. But cysts can grow large, sometimes exceeding 10 cm, and at that size they can physically change the shape of your abdomen. A large cyst may cause visible bloating, a sense of fullness or heaviness in the belly, or a noticeable asymmetry where one side of the lower abdomen looks more swollen than the other. The discomfort often feels like a dull ache below the bellybutton, sometimes shifting to one side. Sharp pain can occur if a large cyst twists or ruptures.
Size Thresholds That Guide Treatment
For premenopausal women, simple cysts smaller than 5 cm don’t require routine follow-up. They’re common, usually functional, and typically resolve without treatment. Cysts between 5 and 7 cm are monitored with repeat ultrasounds every three to six months to track whether they’re growing or shrinking. Cysts larger than 7 cm are generally considered for surgery, largely because a cyst that size increases the risk of ovarian torsion, where the ovary twists on itself and cuts off its own blood supply.
For postmenopausal women, the thresholds are more cautious. Simple cysts under 5 cm with normal blood markers are considered low risk and monitored with routine scans. Any complex, persistent, or symptomatic mass in a postmenopausal woman is typically evaluated for surgical removal, since the chance that a cyst is something more serious increases after menopause.