Endometriosis happens when tissue similar to the lining inside the uterus grows in places where it doesn't belong. During each menstrual cycle, this tissue responds to hormones just like the uterine lining. It can swell and bleed, causing irritation, inflammation, and scar tissue.
Some women have only mild symptoms, while others experience severe pain that affects work, school, relationships, or daily life. In some cases, endometriosis may also make it harder to become pregnant.