If you usually have a regular period, once every 30 days or so, and you get your period on the 21st, the 13th is the theoretical safe period. General ovulation in the next menstrual period about 14 days before the day of ovulation, 5 days before ovulation to 4 days after the day of ovulation is the period of ovulation, a menstrual cycle in addition to ovulation and menstruation outside the time for the theoretical safe period, according to this method of projection, the 21st come to menstruation, the 13th is the theoretical safe period. But ovulation is susceptible to external factors in advance or delayed, the safe period is only relatively safe, not absolutely safe, it is not recommended to use safe period contraception, the failure rate is relatively high. The failure rate is relatively high. There is still a possibility of pregnancy if you have sex during the safe period, if you have sex, you still need to consult a doctor to determine whether you are pregnant after your period is delayed. If you have any other questions, it is recommended that you consult a medical professional for guidance.