Because there are obvious individual differences in women’s menstrual cycles and the timing of menstruation, it is impossible to determine exactly how many days after menstruation is clean is the safe period. To understand the safe period, we must first know how to calculate the ovulation period. A woman’s menstrual cycle is usually 21-35 days, and the 14th day before the first day of the next menstrual period is the day of ovulation, and the 3 days before and 5 days after the day of ovulation are the days of easy conception according to the survival time of egg cells and sperm. For example, if the first day of your last period is March 1, your next period should be on March 30, so 14 days before that, March 16 should be the day of ovulation, and the fertile date should be from March 13 to March 21, except for the safe period. This method is only suitable for women whose menstrual cycle has always been accurate, but even for such women, the method of safe period contraception is not reliable. It is recommended that women should not use this method of contraception, but should choose a truly safe method of contraception that suits their specific situation, because ovulation is fortuitous and can occur early or late due to emotional, environmental and health factors.