If menstruation is relatively regular, the safe period of seven days before and eight days after is credible, if menstruation is not regular is not credible, but it is not recommended to use contraception based on the safe period. If menstruation is regular, ovulation is generally more regular, clinically from 5 days before and 4 days after the day of ovulation, together with the day of ovulation, a total of 10 days is called ovulation. The rest of the time is non-ovulatory, and the chances of getting pregnant during non-ovulatory intercourse are relatively small. If the menstrual cycle is 28 days, minus the 10 days of ovulation and 5 days of menstruation in between, the safe period will be shorter in this case. If the menstrual cycle is 35 days, the safe period will be longer. Most women do not have uncomfortable symptoms on the day of ovulation, cannot accurately calculate the ovulation period, or may have irregular menstruation due to various factors, so it is not recommended to use contraception based on the safe period of the first seven days and the last eight days.