The menstrual cycle is the set of physiological phenomena, occurring most often periodically (approximately one month), which prepare the body of the woman for a possible fertilization.
The process signal of the end of one cycle and the beginning of a new one is menstruation which is explained by the release of the unfertilized ovule with the placenta precursor (which is a blood).
In the case of fertilization, the menstrual cycles are "paused", and "resumed" after the childbirth.