The tube catches the egg and draws it inside. If the egg is fertilised it may embed in the lining of the womb, the endometrium, and a pregnancy has commenced. If it is not fertilised, it is lost together with the endometrium, as the period at the end of the month. Endometriosis also goes through a cycle each month and grows under the influence of the hormone oestrogen. It will also have a period each month, but there is no escape for the blood which remains, and irritates the surrounding tissues.
Several theories exist as to how endometriosis begins.
During a period most of the menstrual blood comes out through the vagina. In women some blood also passes backward through the fallopian tubes into the abdominal cavity. Contained within the menstrual blood are fragments of endometrium, and these can seed and grow in the peritoneal cavity. We do not know why these cells seed in some women and not in others. This theory is called retrograde menstruation.