Does the endometrial tissue, blood and other fluids expelled from the uterus through the vagina in women and some viviparous females in reproductive age, when they are not pollinated. The length of the menstrual cycle varies between species, the human is approximately 28 days, from where comes its name, is in latin mensis "month", more precisely by the lunar month (as an evolutionary mechanism, primitive women synchronising their days ferti (them with the new moon, when the males stayed at home and did not come out to hunt). Although in Greek there is a voice for 'Moon' that sounds like mena, philologists do not associate them, although they represent a common etimo from a proto Indo-European language.