One of the theories proposed to explain the causes of endometriosis is the theory of retrograde menstruation. According to this theory, endometrial cells could reach the abdominal cavity through the fallopian tubes. To do so, these cells would carry a retrograde flow instead of exiting through the vagina.
However, retrograde menstruation occurs in many women and only 15% develop endometriosis. Moreover, this theory does not explain cases of endometriosis that do not occur in the abdominal cavity or cases in men or in girls who have not yet menstruated.