Well it’s actually not possible to get pregnant during your periods. This is because the egg is released only fourteen days after the first day of the period. That is how the normal human menstruation cycle works. So even if you have unprotected sex during your period the sperm won’t survive for 14 days in the womb.
It is possible to get pregnant during the period especially with women who have very irregular cycles. There are women who release eggs anytime during the 28 day cycle so the chances of them getting pregnant are very high. Hence such women have to be extra careful.
Generally it is very difficult to get pregnant one day before or during your period but sometimes women confuse other kinds of bleeding as periods. This can increase your chances of getting pregnant. So you do need to exercise caution and practice safe sex especially if you are not sure if the bleeding is menstrual bleeding.