Endometriosis is a common condition in which small pieces of the lining of the womb (the endometrium) are found outside the womb. This could be in the fallopian tubes, ovaries, bladder, bowel, vagina or rectum.
Endometriosis commonly causes pain in the lower abdomen (tummy), pelvis or lower back. It may also lead to fertility problems. However, some women have few or no symptoms.
The exact cause of endometriosis is not clear however, hormone levels seem to be a feature.
The endometriosis cells behave in the same way as those within the womb, so every month they grow during the menstrual cycle and are shed as a bleed.
Normally prior to a period, the endometrium thickens in preparation for a fertilised egg in response to a release of the hormone oestrogen. When pregnancy does not occur, the lining breaks down and leaves the body during the period.
Endometriosis tissue anywhere in the body will go through the same process of thickening and shedding however, it has no way of leaving the body. This leads to pain, swelling and sometimes damage to the fallopian tubes or ovaries, resulting in fertility problems.