Growths of endometriosis are benign (not cancerous). But they still can cause many problems. To see why, it helps to understand a woman's menstrual cycle. Every month, hormones cause the lining of a woman's uterus to build up with tissue and blood vessels. If a woman does not get pregnant, the uterus sheds this tissue and blood. It comes out of the body through the vagina as her menstrual period.
Patches of endometriosis also respond to the hormones produced during the menstrual cycle. With the passage of time, the growths of endometriosis may expand by adding extra tissue and blood. The symptoms of endometriosis often get worse.
Tissue and blood that is shed into the body can cause inflammation, scar tissue, and pain. As endometrial tissue grows, it can cover or grow into the ovaries and block the fallopian tubes. Trapped blood in the ovaries can form cysts, or closed sacs. It also can cause inflammation and cause the body to form scar tissue and adhesions, tissue that sometimes binds organs together. This scar tissue may cause pelvic pain and make it hard for women to get pregnant. The growths can also cause problems in the intestines and bladder.
This answer is based on source information from The National Women's Health Information Center.