Tissue similar to the endometrium implants outside the uterus — most often on the ovaries, the pelvic peritoneum, the ligaments behind the uterus, and sometimes the bowel or bladder. It responds to the hormonal cycle, bleeding each month into tissues that cannot clear it, which causes inflammation, scarring and adhesions.
The pain is characteristic: severe period pain that worsens over years, pain during or after intercourse, and in advanced disease pain with bowel movements or urination during periods. It is frequently dismissed as normal, and the delay between first symptoms and diagnosis is measured in years in every health system studied.
Fertility is reduced through several mechanisms — distorted pelvic anatomy from adhesions, damage to the ovary from endometriomas, an inflammatory environment hostile to eggs and embryos, and in some women reduced ovarian reserve. Yet many women with endometriosis do conceive, and the relationship between disease severity and fertility is not straightforward.
Adenomyosis, where the same tissue grows within the muscle of the uterus itself, frequently coexists. It causes heavy painful periods and an enlarged tender uterus, and it appears to reduce implantation success independently.