Especially for young girls, ovulation can be irregular. It is even possible (though much less so) to get pregnant on your period. Yes, ovulation usually happens at around day 14 of the woman's cycle (after her period ends), but this is no guarantee. Also, sperm can live for a few days inside the woman's body, so if there isn't an egg there on the first day, they can wait for a few more days.

Bottom line: use birth control if you don't want to get pregnant or just don't have sex. Otherwise, you will have an unwanted pregnancy, or at the very least, a scare.