(menses) n. the discharge of blood and of fragments of endometrium from the vagina at intervals of about one month in women of child-bearing age (see menarche, menopause). Menstruation is that stage of the menstrual cycle during which the endometrium, thickened in readiness to receive a fertilized egg cell (ovum), is shed because fertilization has not occurred. The normal duration of discharge varies from three to seven days. In anovular menstruation, discharge takes place without previous release of an egg cell from the ovary. Vicarious menstruation is bleeding from a mucous membrane other than the endometrium when normal menstruation is due. Retrograde menstruation is the backflow of blood and endometrial cells through the Fallopian tubes (see endometriosis). See also amenorrhoea, dysmenorrhoea, menorrhagia, oligomenorrhoea.