Endometriosis in plain English: what it is, where it occurs, and why it hurts
What endometriosis is (and isn’t)
Endometriosis is a condition where tissue similar to the lining of the womb grows in places it shouldn’t. These growths—often called lesions—can behave a bit like the womb lining across the menstrual cycle. That activity and the surrounding inflammation can irritate nerves, which is one reason pain can be so intense for some women.
It isn’t an infection, it isn’t cancer, and it isn’t “just bad period pain”.
Where it can occur
Common sites include the pelvic peritoneum (the thin tissue lining the pelvis), the ovaries (sometimes forming cysts known as endometriomas), the ligaments behind the uterus and the space between the uterus and rectum. Less commonly, it can involve the bladder or bowel surfaces, the diaphragm, or surgical scars. Location doesn’t always predict symptom severity—small lesions can be very painful, while some extensive disease causes few symptoms.
Why it hurts
Pain is usually driven by a mix of factors:
- Inflammation: lesions can trigger local immune responses and release inflammatory chemicals.
- Nerve changes: over time, nerve endings may grow into lesions and surrounding tissue, amplifying pain signals.
- Cycle effects: hormone fluctuations can change swelling and sensitivity in affected areas.
- Central sensitisation: for some women, the nervous system becomes more reactive over time, so pain feels stronger and lasts longer than the original trigger would suggest.
Symptoms (a spectrum, not a checklist)
Many women report pelvic pain (during periods or throughout the cycle), pain with sex, bowel or bladder discomfort, bloating, fatigue, and difficulty with fertility. Symptoms vary widely and can overlap with other conditions, which is one reason recognition can take time.
Staging and what it means
You may see references to “Stage I–IV”. Staging describes what surgeons see (number, depth and location of lesions and adhesions). It does not neatly predict pain or fertility impact. Think of it as a map for clinicians rather than a score of how “bad” someone feels.
Endometriosis vs adenomyosis
Adenomyosis is a related but different condition where tissue similar to the womb lining grows into the muscular wall of the uterus. It often causes heavy, painful periods and an enlarged, tender uterus. Some women have both.
What we know—and what remains uncertain
Research points to a complex picture involving genetics, immune responses, hormones and nerve growth. There isn’t a single cause or a single presentation, which is why experiences differ so much from one woman to another. What’s consistent is the impact on day-to-day life, study, work and relationships—something health services and communities are increasingly recognising.
Educational only: This article does not provide medical advice. For diagnosis or treatment, please speak to a qualified healthcare professional.