They get confused for understandable reasons. Both are common. Both affect periods. Both are associated with difficulty conceiving. Both are diagnosed years later than they should be, and both are routinely dismissed before that.
They are not variations of one thing. They affect different tissue, they are found by different means, and the symptom that separates them most reliably is one that PCOS does not have among its criteria at all.
The one-line version
PCOS is an endocrine and metabolic condition. It is defined by how the ovaries function — ovulation, androgens — and it brings metabolic features with it. Two of three criteria, with mimicking conditions excluded.
Endometriosis is tissue in the wrong place. Tissue similar to the lining of the uterus grows outside it, and responds to the cycle where it sits. Its dominant feature is pain.
One is a question about function. The other is a question about where tissue is.
Side by side
| PCOS | Endometriosis | |
|---|---|---|
| What it is | Endocrine and metabolic condition affecting ovulation and androgens | Tissue similar to the uterine lining growing outside the uterus |
| Dominant symptom | Irregular or absent cycles; signs of androgen excess | Pain — periods, sex, sometimes bowel or bladder |
| Cycle pattern | Typically irregular | Often regular |
| Diagnosed by | Rotterdam criteria, two of three, after exclusions | Symptoms and imaging; laparoscopy no longer first |
| Metabolic screening | Recommended for everyone with the diagnosis, at any weight | Not part of the picture |
| Fertility link | Usually anovulation | Different mechanisms, related to the disease |
Pain is the dividing line, and it is the one that gets lost
Severe period pain is not a diagnostic criterion for PCOS. Endometriosis is characteristically a pain condition — pain with periods, deep pain during sex, and in some cases pain with passing urine or opening the bowels, alongside fatigue.
This matters because of what a diagnosis does to subsequent conversations. Someone diagnosed with PCOS who later develops disabling period pain has a label available to explain it, and a clinician short of time has a reason not to look further. The pain gets attributed rather than investigated, which is the same failure mode as fatigue and produces the same outcome: years.
The rule that avoids it is simple enough to carry. Pain that stops you doing ordinary things is its own finding. It is not a feature of PCOS you have to accept, and raising it as a separate question rather than as part of a PCOS review is what gets it examined.
How endometriosis is diagnosed now, which has changed
This is worth knowing because the widely repeated version is out of date.
Laparoscopy — surgery to look directly — was long described as the diagnostic gold standard. The 2022 ESHRE guideline moved away from that. Imaging, by ultrasound or MRI, is recommended in the diagnostic work-up, and laparoscopy is reserved for cases with negative imaging, or where empirical treatment has not worked or is unsuitable.
Two details within that are easy to miss and both matter to anyone waiting:
Negative findings do not rule it out. Neither a normal examination nor negative imaging excludes endometriosis. Superficial disease is particularly hard to see.
Severity of symptoms does not track the stage. Someone with minimal disease on imaging can have severe pain, and someone with extensive disease can have little. A scan that looks unremarkable is not a verdict on what you are experiencing.
You can have both, and that is the trap
They are separate conditions with separate mechanisms. Neither protects against the other, and both are common enough that the overlap is not unusual.
The practical problem is diagnostic overshadowing — once one condition is on file, new symptoms get read through it. A woman with PCOS presenting with worsening pelvic pain is at risk of having it filed under a diagnosis she already has. A woman with endometriosis whose cycles become irregular may have it attributed to her known condition rather than examined as ovulatory dysfunction.
A diagnosis is a reason to look at new symptoms more carefully, not less. In practice it often works the other way, and knowing that is most of the defence.
Fertility: both, differently
Both conditions are associated with difficulty conceiving, and the mechanisms are not the same.
In PCOS the usual route is anovulation — cycles without an egg released — which is why the guideline has first-line pharmacological answers aimed at inducing ovulation. In endometriosis the mechanisms relate to the disease itself and the appropriate approaches differ.
Which is why “I have been told it may be hard to conceive” is an incomplete sentence until you know which condition is being referred to, and whether both are in the picture.
What to take to an appointment
Separate the pain from the cycle. Two different records answering two different questions: when bleeding started, and when pain happened, how bad, and what it stopped you doing. Pain logged against dates is a pattern. Pain described from memory is “it’s been bad lately”.
Say if cycles are regular. It is informative in both directions — regular cycles make ovulatory dysfunction less likely and do nothing to make endometriosis less likely.
Name the symptom you came in about. If pain is the thing, say so first, before the existing diagnosis gets a chance to absorb it. The order you raise things in changes the appointment more than it should, and that is a fact about ten-minute consultations rather than about any individual clinician.