Answer: When a girl has her period that means she is most likely NOT pregnant. If she does not have her period, she could be pregnant or could become pregnant.
There are some who incorrectly believe women are fertile only when they are menstruating. This is not true. Women are most fertile about two weeks before their period. In some cases, it is hard to predict exactly when a woman ovulates and is fertile; especially in the case of teens, who may have very irregular menstrual cycles. Detailed information is available here.
Do not have intercourse or allow your penis to make contact with her vulva if you are not using birth control unless you want to be parents.