The endometrium is the tissue that lines the inside of the uterus. Endometriosis occurs when this tissue travels outside the uterus.

What is going on in the body?

The female hormones estrogen and progesterone encourage the growth of endometrial tissue during a woman's monthly cycle. If no fertilized egg implants itself in this lining, it is shed as menstrual flow. In a woman with endometriosis, some of the endometrial tissue is found outside the uterus. This tissue also responds to cyclic hormonal signals. However, it cannot be cast off each month. Instead, the cells cause bleeding and scars. Adhesions, or scar tissue, may weld together organs. These include the fallopian tubes and ovaries. This can cause daily or monthly cyclic pain.

Endometriosis often appears in the pelvis or abdominal cavity. Rarely, distant areas like the lungs or brain are affected.

What are the causes and risks of the condition?

The cause of endometriosis is unknown. Several theories have been proposed. It is possible that: