Because they are not simply there to make our lives difficult, understanding the physiological reasons for skin issues is important. Let's immediately shed some light on this subject.
The rules signify the transition to a new cycle.
Menstruation is quite rare 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 fertilized egg. This expulsion is characterized 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 fertilization occurs, the lining is not eliminated: this is why pregnant women do not have periods. Thus, menstruation corresponds to a visible manifestation of the menstrual cycle in women of childbearing age.