Endometriosis (say "en-doh-mee-tree-OH-sus") occurs when cells that are like the cells that line the inside of your uterus grow outside of your uterus. These cells form clumps of tissue called implants. They usually grow on the ovaries, the fallopian tubes, the outer wall of the uterus, the intestines, or other organs in the belly. In rare cases, they spread to areas beyond the belly.
Endometriosis usually isn't dangerous. But it can cause pain and other problems.