Tissues that normally grow inside the uterus (endometrium) may sometimes grow outside the uterus. This is termed as endometriosis. The main symptoms are pelvic pain and infertility. Nearly half of those affected have chronic pelvic pain, while in 70% pain occurs during menstruation. Pain during sex is also common. Infertility occurs in up to half of the women affected. Less common symptoms may include urinary or bowel symptoms. About 25% of women have no symptoms. Endometriosis can have both social and psychological effects.

Clumps of this tissue called implants may grow on the ovaries, fallopian tubes, outer wall of the uterus, intestine and other organs in the belly. In rare cases they can spread beyond the belly area too.

With each menstrual cycle, the implants go through the same process of growth, break down and bleed that the endometrium goes through. This is why the endometriosis pain may start as a mild discomfort before the menstrual period. If the implant grows in a sensitive area, it can cause constant pain.