The 4 Stages of Endometriosis, Demystified

# The 4 Stages of Endometriosis, Demystified If you've recently been told you have "Stage 2 endometriosis" and immediately went down a Google rabbit hole at midnight — this is for you. Or maybe you're pre-diagnosis, you've heard the staging system mentioned in a forum, and you're trying to figure out whether your pain maps onto a number. Also for you. Here's the thing nobody explains clearly: the stages of endometriosis are genuinely confusing, and not just because the language is dense. They're confusing because the system itself has real, documented problems — problems that doctors acknowledge, that researchers argue about, and that leave a lot of people feeling like the number they've been given doesn't match their lived reality. I was told I had Stage 1 endo after my laparoscopy. Minimal. Which sounds almost fine, doesn't it? Except I'd spent years in pain that was anything but minimal. The staging system didn't capture that, and nobody warned me it wouldn't. So this article is going to do two things. Walk you through what the four stages actually mean — the real, specific anatomy of it, not a vague diagram. And then be honest with you about what staging does and doesn't tell us, because that gap is where a lot of people get lost. The staging system is a tool. It's just not the only tool, and it was never designed to measure your pain.

What Is the Staging System, and Where Did It Come From?

The four-stage classification most gynaes use today was developed by the American Society for Reproductive Medicine — originally in 1979, revised in 1996. It's called the rASRM system (revised ASRM), and it scores endometriosis based on what a surgeon sees during a laparoscopy. That last part matters: you can only be staged during surgery. There is no blood test, no MRI, no ultrasound that gives you a stage. The staging happens when someone is physically inside your body looking at it. The scoring works like this: the surgeon assigns points based on the size, location, and depth of lesions and adhesions — bands of scar tissue that can stick organs together. The points add up to a number that places you in one of four stages: - Stage 1 — Minimal: 1–5 points. Small, superficial lesions. Few or no adhesions.

• Stage 2 — Mild: 6–15 points. More lesions, slightly deeper, possibly near the ovaries.
• Stage 3 — Moderate: 16–40 points. Multiple deep lesions, possible endometriomas (cysts on the ovaries, sometimes called "chocolate cysts"), more major adhesions.
• Stage 4 — Severe: Over 40 points. Extensive deep lesions, large endometriomas, dense adhesions. This is where you might hear about bowel or bladder involvement. The system was primarily designed to predict fertility outcomes — not pain levels, not quality of life, not how disabled you might be on a bad day. That context is everything.

The Part Nobody Leads With:

Stage Does Not Equal Pain This is gonna sound dramatic, but — your stage number might be almost meaningless when it comes to how much you suffer. I mean that clinically, not just emotionally. The research backs it up. Someone with Stage 1 endo can be in agony. Someone with Stage 4 might have relatively managed pain. This isn't rare or anomalous — it's actually well-documented, and it has to do with where lesions are, not just how many there are or how big they've grown. A tiny lesion sitting on a nerve — the pudendal nerve, for instance — can cause pain that's completely out of proportion to what the rASRM score would suggest. A large endometrioma on an ovary might cause less day-to-day pain than a small implant in the cul-de-sac (the space between your uterus and rectum, which is densely innervated). The staging system doesn't account for nerve involvement at all. It doesn't account for the central sensitisation that develops in people who've been in pain for years — where your nervous system essentially gets rewired to pain signals. It doesn't account for whether you also have adenomyosis, which often travels alongside endo and contributes significantly to symptoms. So when someone gets a "minimal" stage and feels dismissed — there's a real structural reason that's happening. The tool wasn't built to see what they're experiencing.

What the Four Stages Actually Look Like in Practice

Let me try to make this more concrete, because "lesions" and "adhesions" can feel very abstract when you're the one living in the body.

Stage 1 and 2:

What "Mild" Actually Means At Stage 1 and 2, you might have what surgeons describe as superficial implants — small spots of endometrial-like tissue on the surface of pelvic structures. The peritoneum (the membrane lining your pelvic cavity), the ovaries, the fallopian tubes. These lesions can be clear, red, or dark. The older assumption was that the dark, "powder burn" lesions were the classic presentation — but we now know clear and red lesions are often more active and more painful ¹. Minimal adhesions at this stage might mean a small amount of scarring, or none at all. "Mild" staging does not mean mild symptoms. Worth saying twice.

Stage 3:

When Endometriomas Enter the Picture Stage 3 is where you start to see endometriomas — cysts that form on the ovaries when endo tissue bleeds into them and the blood becomes trapped. They're called chocolate cysts because of what old blood looks like. (I know. The naming conventions in gynaecology are something.) Endometriomas can affect ovarian reserve — the quantity and quality of eggs. This is one of the reasons endo is associated with fertility challenges, though the relationship is complicated and not deterministic ⁶. Adhesions at Stage 3 can start to involve the bowel or bladder, which is when you might notice symptoms like pain when you go to the bathroom, especially around your period, or urgency and frequency with urination.

Stage 4:

Deep Infiltrating Disease Stage 4 is the one that tends to cause the most fear when people read it on their discharge letter. It's worth being honest about what it means and what it doesn't. Severe endo, by the rASRM definition, involves extensive deep infiltrating lesions — tissue that has grown more than 5mm into the structures it's attached to. Dense adhesions. Often major involvement of the bowel, bladder, ureters, or all of the above. Endometriomas are usually present. This is the stage where you might hear words like "bowel resection" or "ureteric stenting." That's real, and it's frightening to hear. But Stage 4 doesn't automatically mean surgery of that magnitude — it depends on where exactly the disease is and how it's affecting your organs. What Stage 4 does mean is that you need a specialist. Not just a general gynaecologist. Someone who operates in a dedicated endo centre with access to a colorectal surgeon and a urologist if needed, because this level of disease often requires a multi-disciplinary team.

Why Are There Only Four Stages?

And Are They Enough? Honestly — a lot of researchers think they're not. The rASRM system has been criticised for decades for failing to correlate with pain or quality of life. Other classification systems exist: the Enzian classification was developed specifically to score deep infiltrating endo that the rASRM misses. The #ENZIAN system is more detailed about posterior compartment disease (behind the uterus, near the bowel). Some surgeons use both. There's also the Endometriosis Fertility Index (EFI) for people trying to conceive, which tries to factor in functional ovarian reserve alongside the rASRM score. None of these systems are perfect. The field knows this. The staging conversation is genuinely evolving.

What the research actually says

What the Research Actually Shows

The honest version: the research on staging is a mix of "this is useful" and "this has real limits," and it's worth knowing both. Endometriosis affects an enormous number of people globally. As one 2022 BMJ review puts it,

Endometriosis affects approximately 190 million women and people assigned female at birth worldwide. It is a chronic, inflammatory, gynecologic disease marked by the presence of endometrial-like tissue outside the uterus, which in many patients is associated with debilitating painful symptoms.

Horne AW, Missmer SA — Pathophysiology, diagnosis, and management of endometriosis. BMJ (Clinical research ed.), 2022. View paper →

¹ That scale matters — it means the staging system is being applied across an enormous and varied population of people with very different disease presentations. One of the most persistent problems in endo research is diagnostic delay. A 2017 paper in Fertility and Sterility notes that

Theorized to stem from the phenomenon of retrograde menstruation, the diagnosis of endometriosis is typically delayed by 8-10 years owing to misinterpretation of symptoms as common menstrual cramps in adolescent girls and young women.

Ahn SH, Singh V, Tayade C — Biomarkers in endometriosis: challenges and opportunities. Fertility and sterility, 2017. View paper →

³ By the time someone gets staged, their disease may have been progressing untreated for nearly a decade. Whether staging at that point reflects the full history of the disease — or just a snapshot — is something we can't always know. The link between stage and fertility is better established than the link between stage and pain, but it's still not clean ⁶. People with Stage 1 endo can have major fertility challenges; people with Stage 4 conceive without intervention. Stage is one data point, not a forecast. The search for non-surgical diagnosis is ongoing — and relevant to staging, because if we could identify and characterise endo without surgery, the staging conversation would change entirely ⁴. Right now, we can't. No imaging, no blood marker, no urine test has been validated well enough to replace laparoscopy for diagnosis and staging ³. That's a real gap, and it's one the field is actively trying to close. There's also emerging research on the inflammatory mechanisms underlying endo — including the role of immune dysfunction and microbiota changes — that suggests the disease is more systemic than a simple staging system can capture ². The stage tells you about visible lesions. It doesn't tell you about the inflammatory environment, the nerve involvement, or the central sensitisation that shapes someone's experience of living with this. The bottom line from the research is this: staging is a useful surgical descriptor. It's a less useful predictor of how someone will feel, function, or respond to treatment. Researchers know this. It's why the system keeps being revised and supplemented.

What to do with this

What This Actually Means for Your Life

If you've been staged and you're trying to figure out what to do with that information, here's how I'd think about it. Your stage is a starting point, not a verdict. If you have Stage 1 and you're in major pain, that doesn't mean you're imagining it or that it doesn't "count." It means the current staging system doesn't measure pain — which is a flaw in the system, not in you. Ask your surgeon specifically where your lesions are, not just what stage you are. Location matters more than score for understanding your symptoms. Lesions near the bowel, bladder, or nerves behave very differently from lesions on the ovarian surface. If you weren't given that detail, you can ask for it from your surgical notes. If you have Stage 3 or 4 disease, or if deep infiltrating endo is suspected, ask about referral to a specialist endo centre. General gynaecologists — even good ones — are not always trained to excise deep infiltrating disease safely. This isn't about criticising your doctor; it's about matching the complexity of your disease to the right surgical expertise. Track your symptoms separately from your stage. Use a period or symptom diary — paper, app, whatever you'll actually stick to. Document pain levels, bowel and bladder symptoms, the days you can't get out of bed. This is useful data for appointments and gives you a record that exists independently of a staging number. If you're trying to conceive and you have any stage of endo, a referral to a reproductive specialist with specific endo experience is worth pursuing. The interaction between endo and fertility is complex, and it's not as straightforward as "higher stage = harder to conceive" ⁶. If your stage doesn't match your experience — say so, explicitly. You're not being difficult. You're giving your clinical team information they need.