You can only get pregnant if the sperm can survive until you ovulate. Ovulation only takes place 12 to 16 days before your next period. Sperm will most likely survive 3 days inside you with another 2 days added for just in case he's got wonder-sperm so make it 5 days.
So you can see that the chances of getting pregnant from period sex is largely theoretical for women with normal cycles. Unless you have a totally messed up cycle or cycles shorter than 21 days, it is not likely at all. If you are very young your ovulation can also be a bit erratic just after you started having your periods so that is also a dangerous period of your life to be taking these chances.
After ovulation a fertilized egg will travel along for about another 10 days before it implants in your uterus. About 4 days later you will miss your period if the egg implanted successfully. 7 days after your missed period you can expect to get an accurate pregnancy test if you follow the instructions.