The only time that you can get pregnant are during your most fertile days right around the time of ovulation. Ovulation usually occurs 12-16 days before your next period. And fertilization takes around 10 days, and sometimes the pregnancy hormone takes even longer to build up enough to get a positive result. So basically you can't really tell right away after having sex - it's going to take a while for fertilization, etc. I got a great book called "Taking Charge of Your Fertility" - sounds cheesy, I know, but my husband and I have been trying for about 11 months now and it had some really useful info in there. Good luck!