"The endometrial tissue behaves just like it does when it's inside the uterus, which means it responds to hormones and thickens and bleeds every time you have a period," Dr. Kathy Huang, the director of the Endometriosis Center at NYU Langone Health, told BuzzFeed News. The tissue typically grows on pelvic organs, such as the ovaries, fallopian tubes, bladder, and bowel, Huang says. In rare cases, it can grow on organs elsewhere in the body, such as the lungs or even the brain. The exact cause of endometriosis is unknown.
"Because the blood has nowhere to go and it gets trapped in the body, this can cause inflammation ... so wherever the lesions or endometrial tissue is, there's a lot of scarring and pain, usually during the period," Huang says. So the most common symptoms are pelvic pain, painful periods, pain during intercourse, bowel issues, and excessive bleeding.
The pain associated with endometriosis is not something that goes away after popping some ibuprofen and using a hot water bottle. It's often described as excruciating, persistent, and even debilitating. When endometriosis goes undiagnosed and/or untreated, Huang says, the scarring can become so severe that the areas with lesions hurt all the time, not just during a period. Long-term complications can include infertility, pelvic cysts, "chocolate" cysts on the ovaries, and even bowel obstructions.