Endometriosis doesn’t have one single appearance. During surgery, it can show up as black spots, red blisters, white scars, clear bubbles, or dark blood-filled cysts, depending on how long the tissue has been there and where it’s growing. This wide range of appearances is one reason the condition is so often missed or delayed in diagnosis.
The “Classic” Dark Lesions
The most recognized form of endometriosis is the so-called powder-burn or gunshot lesion. These are black or dark blue-brown spots, typically 1 to 2 centimeters or larger, with a puckered texture surrounded by a white, scarred area. The dark color comes from old, trapped blood that has broken down over time, leaving behind iron deposits in the tissue. These lesions look a bit like someone touched a lit cigarette to the tissue surface, which is how they got their nickname.
For decades, surgeons were trained to look only for these dark spots. That meant many people with endometriosis were told their pelvis looked “normal” during surgery, even when active disease was present in a less obvious form.
Subtle and Early-Stage Lesions
In 1986, researchers recognized that endometriosis also appears as non-pigmented lesions, likely representing earlier stages of the disease. These are small (1 to 3 millimeters) and can look like:
- Red vesicles or flame-like patches: tiny blisters or streaks that resemble small burns on the tissue surface
- White spots: small opaque patches where scarring has already begun
- Clear vesicles: transparent, bubble-like elevations in the tissue lining that can be extremely difficult to spot
These atypical lesions are actually more common than the classic dark spots in younger patients with pelvic pain. Clear lesions in particular can be so faint that surgeons sometimes mistake them for light reflecting off wet tissue. Identifying them requires filling the pelvis with fluid during surgery so the transparent bubbles or “grainy” patches become visible against the surrounding tissue.
Ovarian Endometriomas (Chocolate Cysts)
When endometriosis grows on or into an ovary, it can form a fluid-filled cyst called an endometrioma. These are commonly known as “chocolate cysts” because they’re filled with thick, dark brown fluid, essentially old blood that has accumulated over many menstrual cycles. The color and consistency genuinely resemble melted chocolate.
From the outside during surgery, an endometrioma often looks like a smooth, dark, rounded mass attached to or embedded in the ovary. On ultrasound, it appears as a well-defined cyst with a distinctive ground-glass pattern inside, which reflects the dense, uniform quality of the old blood. This appearance is recognizable enough that ovarian endometriomas can now be diagnosed through imaging alone, without surgery, according to guidelines from major reproductive medicine societies.
Deep Infiltrating Endometriosis
The most severe visual form is deep infiltrating endometriosis, where tissue grows at least 5 millimeters below the peritoneum (the thin lining that covers pelvic organs and the abdominal wall). Rather than sitting on the surface like other lesions, these form firm nodules that burrow into surrounding structures. About 14% of patients in one large study of over 1,100 people had this form.
Deep nodules feel hard and fibrous to the touch, more like scar tissue than normal organ tissue. They can distort the shape of the organs they invade, pulling structures together with dense adhesions. Of those with deep disease, roughly 90% had nodules in the area between the rectum and the vagina. These nodules can also appear on the bladder, the bowel wall, or the ligaments that support the uterus.
Where Lesions Appear Most Often
Endometriosis has a strong preference for certain locations in the pelvis. In a study of 1,101 patients, the ovary was the most common site, affected in about 67% of cases. The ligaments behind the uterus were involved in roughly 46%, followed by the tissue next to the ovary (32%), the space between the uterus and rectum (about 30%), and the bladder (21%). Less commonly, lesions show up on the diaphragm, the appendix, or surgical scars from prior procedures.
The pattern matters because location influences symptoms. Lesions near the bladder tend to cause urinary urgency or pain during urination. Lesions on the bowel can cause painful bowel movements, bloating, or changes in stool patterns that cycle with menstruation. Lesions on the ligaments behind the uterus are a common source of deep pain during sex.
What It Looks Like Under a Microscope
A definitive diagnosis requires examining biopsied tissue under a microscope, where a pathologist looks for two key components: glandular tissue and stromal tissue (the supportive cells that normally surround uterine glands) growing outside the uterus. In more established lesions, there are also iron-stained deposits from repeated bleeding.
The challenge is that not every biopsy captures all of these components. In some patients with clearly visible and even severe disease during surgery, biopsies may show only scar tissue or iron staining without the glandular tissue needed for a textbook-perfect confirmation. This is why experienced surgeons sometimes treat based on visual recognition during surgery, even when biopsy results are inconclusive.
Why Appearance Varies So Much
The visual diversity of endometriosis reflects the age and activity of each individual lesion. Fresh, actively bleeding implants tend to appear red or clear. Over time, as blood breaks down and iron accumulates, they darken to brown or black. Eventually, the body’s scarring response turns them white and fibrotic. A single patient can have lesions at all of these stages simultaneously, creating a patchwork of colors and textures across the pelvic surfaces.
This variability is a core reason endometriosis takes an average of 7 to 10 years to diagnose. A surgeon unfamiliar with the full spectrum of appearances may not recognize the subtle clear vesicles, red flame-like patches, or white scars as disease. The condition genuinely looks different from one person to the next, and even from one spot to another in the same pelvis.