The endometrium is the tissue that lines the inside of the uterus (the womb). It is the tissue that is shed each month when women menstruate. Each month it builds up rapidly in preparation for pregnancy, and each month the excess endometrial tissue is sloughed off during menstruation if pregnancy does not occur.
In endometriosis, endometrial cells are found outside the uterus, usually in other parts of the abdomen. These cells respond to female hormones in the same way as the lining of the uterus does. Each month, tissue builds up, breaks down, and sheds. Unlike the cells inside the uterus, the blood and tissue that are shed in the abdomen have no way of leaving the body. They stick to other tissue and sometimes start to divide and multiply. They may grow into other tissue, or form strands that bind organs together. They may create scar tissue, which can be painful. Sometimes the endometrial cells create cysts that can rupture and bleed.
The process sounds a bit like cancer, but endometriosis isn't cancer. However, it may very slightly increase the risk of getting certain cancers. Endometriosis isn't a fatal disease but can be widespread. About 10% to 15% of actively menstruating women between the ages of 25 and 44 have endometriosis. About one-quarter to one-half of infertile women have the disease.