If a girl had sex on her period or right after her period how likely is she to get pregnant?

Possible: Anytime you have unprotected sex, there is a possibility of pregnancy. People are not as predictable as we might like and both sperm and egg have been known to be viable longer than expected. Always use condoms for std protection and another effective birth control method when you have sex, if being a parent is not something you want at this time!

Yes: Pregnancy has been documented from unprotected sex on any day of a woman's cycle, including during menses. It is less common on those days than mid cycle, but it happens. Unless you want to be pregnant, use birth control.