Understanding the physiological reason for menstruation.

Menstruation is relatively uncommon in the animal kingdom. Indeed, we only share this characteristic with a few other species: primates (almost all Catarrhines, some Platyrrhines), bats, a species of shrew, and the Peters' elephant-nosed rat. Each month, the cycle begins with menstruation, when the previous cycle has not resulted in pregnancy. The uterus then sheds the lining that had developed to accommodate a fertilised egg. This expulsion is characterised by a blood-like biological fluid (composed of blood, vaginal secretions, and endometrial cells from the uterine wall), which can vary in volume, and lasts between 3 and 7 days and may be accompanied by pain. Finally, if fertilisation occurs, the lining is not eliminated: this is why pregnant women do not menstruate. Thus, menstruation corresponds to a visible manifestation of the menstrual cycle in women of childbearing age.