Is Pregnancy Possible Post Unprotected Sex?

Doctor me and my gf had slept together on last 11th Dec 2014 and eventhough we hvnt had intercourse really her period isn't starting.. we are too afraid and she is in a bad situation because of tension so is there any way for us to prevent this.. her last period was on 15th Nov and she always had ireegular periods.. our families have accepted but not yet married as we both are completing studies.. what can I do?