Two days before menstruation is the safe period, because if the usual menstrual period is regular, the cycle is 30 days, ovulation is 14 days before menstruation, after the egg is discharged if there is no fertilization, it will die naturally in about 24 hours. In the 2 days before menstruation, the egg has already died, even if the sperm enters the uterine cavity to reach the fallopian tube, there is no egg combination, so it is impossible to get pregnant, this is the safe period. However, if you usually have irregular menstruation, it is difficult to say whether it is a safe period or not, because once there is a delay in menstruation, it is possible that two days before menstruation is the ovulation period. Therefore, to calculate the safe period, clinically it must be regular menstrual cycle can be calculated, if the menstruation is not regular, the calculated safe period is not necessarily accurate.