Here are most of the factors you should consider:
1. What forms of birth control were used. Did you use condoms, is she on any birth control pill or the shot (depo)?
2. How fertile the both of you are. If either you or she has reproductive issues of any kind, that could have an influence.
3. What point in her cycle she is currently in. The closer she is to her most fertile point , the higher the risk of pregnancy.
4. Her pregnancy history. Has she ever been pregnant before?
5. Her gynecological history.
If you’re honestly quite worried and the two of you would not want to go through a pregnancy, and if it has been less than 48 hours since you had sex, go to a Planned Parenthood and get the morning after pill (emergency contraception), which isn’t too expensive. It is two pills, one taken immediately and the other taken twelve hours later, that will lower the chance of pregnancy into the single digits. Do note that it MUST be within 48 hours.
My advice would be; if you aren’t ready to be a father, be very careful with your contraception methods. There are a host of products that range from 70–99.99% effective. USE THEM.