For women with regular menstruation, the safe period is usually 7-10 days before the onset of menstruation. The safe period is the time when ovulation is presumed based on the usual pattern of menstrual cycle, thus avoiding intercourse during ovulation for contraceptive purposes. For women with a 28-day menstrual cycle, the day of ovulation is on the 14th day, and the period from 5 days before to 4 days after ovulation is the ovulation period. The normal regular menstrual cycle for women is 28±7 days, i.e. 21-35 days, less than 21 days for frequent menstruation, more than 35 days for scanty menstruation, there is no safe period for women with frequent menstruation or scanty menstruation, which should be noted. Women’s menstrual cycle is easily affected by environment, life, diet, emotions, etc. Even for women with regular menstruation, the ovulation day can be advanced or pushed back, and the safety period can also change. Therefore, the failure rate of the safety period contraceptive method is high, and other contraceptive methods should be used or combined as much as possible.