You basically can’t get pregnant if you have intercourse when you have your period, but there may be a very small chance of pregnancy in women who have long, heavy periods.
In the case of regular menstruation, ovulation occurs about two weeks before the next period, and the probability of pregnancy is greater on the day of ovulation and the 3-4 days before and after it. At other times, such as during menstruation, ovulation is less likely to occur, so the probability of pregnancy is lower, and you will basically not get pregnant.
However, the fact that ovulation is less likely during menstruation does not mean that 100% of women do not ovulate. For women who have shorter menstrual cycles and longer periods, there may already be dominant follicles growing and developing towards the end of the period, and if you have sex without contraception, you may be able to conceive once you ovulate within 24 hours of having sex.
Sexual intercourse is usually not recommended during menstruation, as it may cause gynecological inflammation and should be avoided as much as possible.