The absolute safe period is defined as 4 days after menstruation and 5 days before the onset of menstruation. From a contraceptive point of view, the menstrual cycle is divided into three phases: the absolute safe period, the safe period and the risk period. The 3-5 days before and after ovulation have the highest chance of pregnancy and are the danger period for women on birth control. The first 7 days and the last 8 days of the menstrual cycle, when there is usually no egg to be ovulated, are not likely to be pregnant and are called the safe period. 4 days after menstruation and 5 days before menstruation, when you are in the early follicular development phase or the end of luteal atrophy, is the absolute safe period. However, this method is only applicable to women who have regular menstruation, because ovulation can be affected by the external environment, weather conditions, one’s emotions and health status, so contraception through the safe period is not absolutely safe. For women with unstable menstrual cycles, ovulation can be monitored using ovulation detection test strips or ultrasound. For women who want to use contraception, condoms or IUDs are more effective.