What is the menstrual cycle? The menstrual cycle refers to the time span of a woman’s menstrual period, usually from the first day of one menstruation to the first day of the next. A normal cycle is about 28 days, but some women have cycles of different lengths, such as 21 days or 35 days. How to record menstrual cycles In order to correctly predict menstrual cycles, women need to record the start and end time of each menstrual period, as well as the amount of menstrual blood. Today, there are many mobile apps that help women