Endometriosis doesn’t have one single appearance. During surgery, it shows up as spots, patches, and nodules that range in color from bright red to deep black, and its look changes depending on where it grows and how long it’s been there. On imaging like ultrasound or MRI, it has its own distinct set of visual markers. Here’s what doctors actually see.
What Surgeons See During Laparoscopy
Most endometriosis is identified during laparoscopy, a minimally invasive surgery where a camera is inserted through a small incision near the navel. The classic lesions are black or dark bluish spots on the pelvic lining, sometimes called “powder-burn” lesions. These dark spots get their color from blood pigments that accumulate over time as the misplaced tissue repeatedly bleeds with each menstrual cycle. Brown and deep red lesions form through the same process and are among the easiest for surgeons to recognize.
But endometriosis doesn’t always look so obvious. Subtler forms include red flame-like patches, white opacification of the peritoneal lining, yellowish-brown patches, and even small holes or defects in the peritoneum itself. These “faint” lesions are easier to miss and harder to distinguish from other conditions like chronic inflammation or scarring from previous surgeries. Studies using biopsy confirmation found that red lesions were the most likely to contain active endometriosis under the microscope, followed by black lesions. White lesions were the least likely to be confirmed as endometriosis histologically, suggesting they may represent older, burned-out disease.
Ovarian Endometriomas (Chocolate Cysts)
When endometriosis grows on or inside an ovary, it can form a fluid-filled cyst called an endometrioma. These are commonly nicknamed “chocolate cysts” because the fluid inside is thick, dark brown, and syrupy, the result of old blood that has broken down over many cycles. The cysts can range from small to quite large. Once an endometrioma reaches about 6 cm, it raises the risk of ovarian torsion, where the ovary twists on itself and loses blood supply.
On ultrasound, endometriomas have a characteristic “ground-glass” appearance: a uniformly hazy, low-level echo pattern inside the cyst that reflects the old hemorrhagic debris. They’re typically rounded with regular margins, appear darker than surrounding tissue, and show minimal blood flow on Doppler imaging. They’re often found on both ovaries.
Deep Infiltrating Endometriosis
The most aggressive form, deep infiltrating endometriosis, invades more than 5 mm below the surface of the tissue it affects. Rather than flat spots, it forms firm nodules that can feel like hard lumps embedded in tissue. These nodules most commonly develop on the ligaments behind the uterus, the space between the rectum and vagina, and the area behind the cervix. Less commonly, they appear on the bladder, bowel wall, or even in the chest cavity and around nerves.
Deep nodules tend to trigger significant scarring and fibrosis, making the surrounding tissue stiff and thickened. In the bowel wall, they appear as irregular, dark thickenings that invade into the muscle layer. When deep disease is extensive, it can create what surgeons call a “frozen pelvis,” where organs that normally slide freely against each other become locked together by dense scar tissue.
Adhesions and Scar Tissue
Endometriosis almost always produces some degree of adhesions, bands of scar tissue that stick organs and tissues together. These range from thin, filmy veils that are nearly transparent to thick, rope-like bands that dramatically distort pelvic anatomy. In advanced disease, adhesions can pull the uterus backward, trap the ovaries against the pelvic wall, kink loops of bowel, and completely seal off the space behind the uterus (the pouch of Douglas). On MRI, these adhesions appear as spiky, dark strands that blur the normal fat spaces between organs and pull structures out of their expected positions.
What It Looks Like on MRI
MRI is particularly useful for mapping deep endometriosis before surgery. Deep implants show up as dark nodules on certain MRI sequences, especially in the tissue behind the cervix, where they interrupt the normal bright line of the uterine surface. Endometriomas appear as cysts with a distinctive bright signal on specific imaging sequences due to their blood content. Adhesions show as dark, spiculated strands pulling on organs, sometimes with the uterus displaced backward or the ovaries pulled into unusual positions.
Endometriosis Outside the Pelvis
In rare cases, endometriosis appears on the diaphragm, liver surface, or even in the lungs. On the diaphragm, it can look like puckered black lesions similar to pelvic implants, or yellowish-brown “café au lait” patches from iron deposits left by old bleeding. Some diaphragmatic lesions are superficial, sitting on the peritoneal surface, while others form deep nodules that partially or fully penetrate through the diaphragm muscle. New blood vessels often form around these lesions, visible as a web of tiny vessels at the edges of the implant. On MRI, diaphragmatic endometriosis may appear as bright nodules, flat plaques, or even small “air-filled bubble” lesions.
Diaphragmatic endometriosis is notoriously hard to detect on imaging before surgery. In one single-center study, MRI identified it preoperatively in only two patients, with most cases discovered during laparoscopy.
What Confirms the Diagnosis
What you see during surgery can strongly suggest endometriosis, but the definitive answer comes from examining biopsied tissue under a microscope. Pathologists look for at least two of three features: endometrial-type glands (the same glandular tissue that lines the uterus), endometrial-type stroma (the supportive tissue surrounding those glands), and evidence of chronic bleeding, specifically immune cells stuffed with iron from broken-down blood. Finding these elements in tissue that’s outside the uterus is what separates a confirmed diagnosis from a visual impression.
This distinction matters because several conditions mimic endometriosis visually. Dark spots can turn out to be reactions to old suture material, burns from previous surgical instruments, tiny blood vessel tumors, or even cancer metastases. Studies show that visual diagnosis alone has meaningful error rates, particularly for white and subtle lesions, which is why biopsy remains the gold standard.
How Severity Is Classified
Surgeons score what they see using the revised American Society for Reproductive Medicine (rASRM) classification system, which assigns points based on the size, depth, and location of implants along with the extent of adhesions. The total score places disease into one of four stages: Stage I (minimal, 1 to 5 points), Stage II (mild, 6 to 15 points), Stage III (moderate, 16 to 40 points), and Stage IV (severe, over 40 points). Stage IV typically involves large endometriomas on one or both ovaries, multiple deep implants, and thick adhesions binding pelvic organs together.
One important caveat: the stage doesn’t reliably predict symptoms. Some people with Stage I disease have debilitating pain, while others with Stage IV have relatively mild symptoms. The staging system describes what the disease looks like anatomically, not how it feels.