Anytime you stick your penis in a girl without protection, a pregnancy is a possible outcome. That said, your odds are pretty low. I know it can be scary worrying if your girlfriend is pregnant, I've been there too. The important thing is to keep calm, and above all, learn from the situation. Read up on how a pregnancy happens, and the ways you can avoid it. And remember, if you are not going to practice abstinence, always use multiple forms of protection. Good luck.