In endometriosis, small or large patches of endometrial tissue, which is usually located only in the lining of the uterus (endometrium), appear in other parts of the body. How and why the tissue appears in other locations is unclear.

Common locations of misplaced endometrial tissue include the ovaries and ligaments supporting the uterus and, less commonly, the fallopian tubes. But the misplaced tissue may also appear in other locations in the pelvis and abdomen or, rarely, on the membranes that cover the lungs or heart.

The misplaced endometrial tissue can irritate nearby tissues, causing bands of scar tissue (adhesions) to form between structures in the abdomen. The misplaced tissue can also block the fallopian tubes, causing infertility.