What is menstrual period? Female menstrual period refers to the three periods of premenstrual period, menstrual period and postmenstrual period. A normal female menstrual cycle usually lasts about 28 days, but this number will vary depending on individual differences. The menstrual period begins with the onset of menstruation and ends with the next menstruation. How to Calculate Your Menstrual Cycle Knowing your menstrual cycle is very important, especially when it comes to family planning and predicting your menstrual cycle. Here are several methods to calculate menstrual cycles: 1. Calendar method: record the date when menstruation begins, and continue to record the date when the next menstruation begins,