If you have intercourse before your period, you will not get pregnant because ovulation occurs about 14 days before your period and the egg will die naturally if it is not fertilized. If you have your period on the 30th of each month, but it is not regular, you may delay it until the 14th of the month, so if you have intercourse before your period, your chances of getting pregnant will increase. In addition, if the period is irregular, ovulation may also change accordingly. In clinical practice, if you have intercourse before the said period and think that it is before the period, you may also be just in time for ovulation, and it is recommended to give contraception.