2. Retrograde menstruation

In the 1920’s, Dr. John Sampson supported a theory where he explained that it is possible menstrual tissue can flow in reverse through the fallopian tubes and lands on the pelvic organs where it may stay and grow. Despite this discovery, researchers found that 1 in 9 women have retrograde flow and not all of them develop endometriosis which led physicians to figure that something else may be triggering the disease.