Endometriosis affects 176 million women worldwide, and 1 in 10 women of reproductive age. While the disease has been intensively studied over the last century, research has not proven a definite cause. There are, however, several strong theories regarding how and why endometriosis occurs.

One of the oldest and most likely explanations is known as “retrograde menstruation.” This theory says that some menstrual and endometrial tissue debris flows backward from the uterus through the fallopian tubes and into the pelvis during a menstrual period. The tissue will then implant and grow where it lands in the pelvis. This theory does have some merit, but retrograde menstruation cannot be the sole cause of endometriosis. Virtually all women experience retrograde menstruation, yet only some develop endometriosis.

Another possibility is what’s called “coelomic metaplasia.” This theory proposes that the coleomic (abdmonial) cavity possesses primitive cells that can change structure and function to become endometrial cells when they are influenced by certain conditions. This theory is based on embryologic studies demonstrating that all pelvic organs, including the endometrium, come from cells lining the abdominal cavity and these cells have the ability to become whatever they want to become.

A third theory is “direct transplantation.” Simply put, it suggests that endometriosis can develop in episiotomy, Cesarean section, and other scars after abdominal surgery—a result of the operation scar becoming contaminated with endometrial tissue.

When endometriosis develops in rare sites like the lungs, brain and other organs distant from the pelvis, a probable explanation is that endometrial cells are transported around the body via the bloodstream and lymphatic system (lymphatic vessels transport fatty substances). This is known as the “vascular theory.”

Finally, evidence suggests that women with endometriosis may have an immune defect that promotes the development or progression of the disease. While all women have some endometrial tissue that grows outside the uterus, if their immune systems are healthy, this “misplaced” tissue is detected and destroyed. According to the “immune dysfunction” theory, women who develop endometriosis may have an immune system that is unable to destroy the misplaced endometrial tissue.