Endometriosis is the abnormal location of elements of the endometrium outside the uterine cavity. In women who have a regular menstrual cycle, the mucous membrane builds up in the uterine cavity for three weeks and then, under the influence of hormones, it flakes off during menstruation. In some women, unfortunately, some of the endometrial cells along with the menstrual blood end up in the fallopian tubes and escape into the abdominal cavity.