With a normal 30-day menstrual cycle, the safe period for menstruation on the 13th is the 14th to the 25th, and the 5th to the 13th of the following month, both of which are relatively safe periods. Women with normal menstrual cycles, the day of ovulation is the 14th day before menstruation, the 14th day of the first five days and the next five days belong to the danger period, so the 13th menstruation, the day of ovulation should be the 30th, then the danger period is the 25th ~ the next month, the 4th of the 10 days of this period is the danger period, most of the women are in the danger period to ovulate; exclude these 10 days, the rest of the time is the safe period, the chances of pregnancy is relatively small. Since menstruation is affected by many factors, such as staying up late, environmental changes, emotional changes, nervousness, etc., which may lead to early or staggered ovulation, the safe period contraceptive method is the least safe. If there is any menstrual abnormality, it is recommended to go to the hospital in time for examination and follow the doctor’s advice to deal with the treatment.