In rare cases they can spread outside the abdomen and affect other organs, such as the lungs. Like the endometrium itself, the transplanted tissue responds to the hormones estrogen and progesterone by thickening and may bleed every month. But because the transplanted tissue is embedded in other tissue, the blood it produces cannot escape and ends up irritating the surrounding tissue, causing cysts , scars and adhesions to form. These can eventually bind the reproductive organs together so that they move as one mass when manipulated by a physician. Cases of endometriosis are classified as minimal, mild, moderate or severe, depending on the size of the lesions and how deeply they reach into host organs.
Symptoms vary, and some women have no noticeable symptoms at all.
Others experience abnormally heavy and painful periods and sharp, deep abdominal pain during intercourse. Endometriosis may make it difficult to become pregnant. In women who are able to conceive, symptoms may diminish or disappear during pregnancy, but they may return after delivery. Endometriosis is estimated to affect between 5 percent and 20 percent of women of reproductive age. The average age at diagnosis is between ages 20 and 29.