The main reason is that this time the egg has already died, because ovulation is in the 14 days before menstruation, the egg is discharged after the survival time is only 24 hours, and then will die naturally, in the 4 days before menstruation, the egg has already died, the sperm even if it reaches the fallopian tubes, there is no egg combination, so the chance of getting pregnant is almost zero. If you have a bad month or a change in the environment, it is possible that the endocrine changes, so that the menstrual changes, menstrual changes will lead to changes in ovulation, so in order to be careful, it is best to use a condom to prevent intercourse, to achieve the purpose of complete peace of mind.