Endometriosis has four stages, numbered I through IV, ranging from minimal to severe. These stages are defined by the American Society for Reproductive Medicine (ASRM) based on a point system that scores the location, size, and depth of endometrial tissue growing outside the uterus. The staging tells you how much disease is physically present, but it does not reliably predict how much pain you’ll experience.
How the Four Stages Are Determined
Staging uses a point system. During a surgical procedure called laparoscopy, a surgeon examines the pelvis and scores what they find: where the tissue implants are, how deep they go, whether ovarian cysts are present, and how much scar tissue (called adhesions) has formed. The points are then added up to assign a stage.
- Stage I (Minimal): 5 points or fewer
- Stage II (Mild): 6 to 15 points
- Stage III (Moderate): 16 to 40 points
- Stage IV (Severe): More than 40 points
This scoring system was designed to give surgeons a standardized way to document and communicate how much disease they see. It is not a scale of how sick someone is or how much they’re suffering.
Stage I and Stage II: Minimal and Mild
In Stage I, the implants are small and superficial, sitting close to the surface of the tissue lining the pelvic cavity. There is little to no scar tissue. This is sometimes called superficial peritoneal endometriosis.
Stage II involves more implants, and they tend to be located deeper inside the tissue. Some scar tissue may be present, but significant inflammation is not typical at this stage. Despite the “mild” label, people with Stage I or II endometriosis can still have debilitating pain. The word “minimal” describes the visible extent of the disease, not the experience of living with it.
Stage III and Stage IV: Moderate and Severe
Stage III is characterized by deeper implants, more scar tissue, and often the presence of endometriomas, which are cysts that form on the ovaries when endometrial tissue grows there. These cysts are sometimes called “chocolate cysts” because they contain old, dark blood. Adhesions become more common at this stage, with bands of scar tissue potentially binding organs together.
Stage IV involves large, deep implants and extensive adhesions. Endometriomas are typically larger or present on both ovaries. The disease may affect the area between the uterus and rectum, and organs in the pelvis can become distorted or stuck together by scar tissue. In severe cases, the bowel, bladder, or the tubes connecting the kidneys to the bladder can be involved.
Why Stage Doesn’t Match Pain
One of the most frustrating aspects of endometriosis is that your stage may have almost nothing to do with your symptoms. A large study of over 1,000 patients published in Human Reproduction found that the association between endometriosis stage and the severity of pelvic symptoms was “marginal and inconsistent.” The researchers did find a slight statistical link between higher stage and period pain, but the actual difference was so small it was clinically meaningless.
This means someone with Stage I can experience severe, life-altering pain, while someone with Stage IV might have relatively mild symptoms and only discover the disease during a fertility workup. The location and type of lesion, nerve involvement, and individual pain processing all play roles that the staging system simply doesn’t capture. If a doctor tells you your stage is “only” I or II, that does not invalidate what you’re feeling.
How Staging Is Done
The official ASRM stage is assigned through surgery, typically laparoscopy. A surgeon inserts a small camera through a tiny incision near the navel, examines the pelvic organs, and scores the findings. Tissue samples are often taken and examined under a microscope to confirm the diagnosis.
Imaging tools like ultrasound and MRI can identify endometriomas and deep tissue involvement, and a confirmed imaging diagnosis is considered valid without surgery. However, imaging often misses superficial implants, so it cannot provide a complete ASRM stage on its own. Many people with endometriosis are treated based on symptoms and imaging without ever receiving a formal surgical stage.
Newer Ways to Describe Endometriosis
The four-stage ASRM system has been around for decades, and many specialists consider it incomplete. It was originally designed with fertility in mind, not pain or quality of life, and it doesn’t do a great job describing deep infiltrating endometriosis, the form that grows into organs like the bowel or bladder.
A newer system called the Enzian classification was developed specifically to map deep disease. It divides the pelvis into compartments: the front (including the bladder), the sides (pelvic sidewall and ligaments), and the back (the rectum and the space behind the cervix). Each compartment is graded on a 1 to 3 severity scale. Enzian also tracks involvement of specific organs, including the bowel above the rectum, the ureters, and even distant sites like the diaphragm or abdominal wall. This gives surgeons a much more detailed picture of complex disease than the traditional four stages alone.
In practice, your medical team may use both systems or neither. Many clinicians describe what they see in plain anatomical terms rather than assigning a formal stage number.
Staging and Fertility
The ASRM staging system was created largely to help predict fertility outcomes, but even for that purpose it has limitations. A separate tool called the Endometriosis Fertility Index (EFI) was developed to fill the gap. The EFI combines your age, how long you’ve been trying to conceive, prior pregnancies, and surgical findings, specifically how well the fallopian tubes, fimbriae, and ovaries function after surgery. It produces a score that predicts the likelihood of non-IVF pregnancy over the following years.
If you’re concerned about fertility, the EFI is a more useful number than your ASRM stage. A person with Stage III endometriosis and good tube and ovary function after surgery can have better fertility prospects than someone with Stage II and significant damage to those structures. Your reproductive endocrinologist can calculate this score after a surgical evaluation.