While the doctors can not pinpoint exactly how far along in pregnancy she is, they can and do estimate it based on the growth rate and size of the baby. They can easily be off by as much as a week (maybe two since we're talking about the pregnancy being half way through already). So if the doctor says she is 5 months pregnant, and you only had sex with her for the 1st time 3 months ago, I'm willing to believe the doctor more than her. Sounds like you need to have a really serious conversation with her, and be willing to take a paternity test to prove the baby isn't yours before you get stuck paying for his collage education.