Endometriosis is caused when cells that should exist in the womb lining migrate to different parts of the body.

These cells, despite being in the wrong place, respond to a woman's normal hormonal changes and can cause internal bleeding.

Sufferers have severe abdominal or back pain, which gradually gets worse as each period approaches.

Complications include reduced fertility, a high risk of ectopic pregnancy and there is a danger of depression among sufferers.

Treatments can include hormones given by mouth, injection, nasal spray or skin patch which may help reduce the activity of the endometrial cells.