I'm no expert, but if the last time you had sex with her was July 18th, then I don't think you are the father. If she were pregnant by you, she would have been at least 17 or 18 weeks pregnant at Halloween, not just 11 weeks. Of course, there is always a chance her dates are off & she's further along than she thinks. If her dates are correct though, then I would guess you're not the father. Hope that helps.
I got pregnant the last weekend in June-My last period was right around the 12th of june. Which would of made me 20 weeks on Halloween. Im now 23 weeks & my due date is March 19th. Sorry hope that helps!
Unless some of her dates are incorrect (due date and weeks along) I'm 99% sure you're not the father. I was 10.5 weeks at Halloween, my due date is May 26. So those tie, but my child was conceived on September 2nd...which is much later than your date of conception.