Endometriosis is a condition when tissues similar to the lining of the uterus grow outside the uterus. It can be found in the fallopian tubes, ovaries, and bowels and even in the lungs.
These tissues behave the same way as the lining of the uterus, i.e. they grow under the influence of oestrogen and even bleed during periods. However, after bleeding from these tissues, the blood gets trapped inside the body. The trapped blood causes inflammation and pain. Over the course of time scar tissues (adhesions) develop.
About 30% of asymptomatic women with otherwise unexplained subfertility will be diagnosed with mild endometriosis if laparoscopy is undertaken.