And Kim is right in her respect because there is always a chance when you have unprotected sex and a slight chance when you have protected sex. The reason being that if you ovulated late (say, right before when you expected your period).
I believe Lance06 meant that you ovulate about 14 days before your period, as any doctor/nurse/medical professional will tell you. So unless you had sex about two weeks before your period, highly unlikely you would be pregnant.
The chance of getting pregnant from having sex right before your period starts is very unlikely. A woman normally ovulates approximately 12-14 days after their first day of their cycle, although every woman is different, I am sure there is a slight chance, but i am going to say not likely!