When a woman menstruates, a flow of blood comes from her womb. Women menstruate once a month unless they are pregnant or have reached the menopause. Lean hard-training women athletes may menstruate less frequently or not at all. + menstruation men·strua·tion Menstruation may cease when a woman is anywhere between forty-five and fifty years of age. when a woman menstruates, usually every month, blood flows from her body (past participle of menstruari, from menstruus , from mensis )