Is it possible to be pregnant for 11 months+ without being overdue?

A year ago my fiance cheated on me a few times with the same person.. when I found out she said to me that she thought she was pregnant. This was January 6th. She then got back together wih her boyfriend of like 7 years or something, and actually got pregnant. She just had her baby yesterday, which would make her begin her pregnancy at like the very end of January or early February. Anyways my fiance said the last time they even did anythign was before Christmas of last year, so that would make it impossible for the baby to be his right? I ask because the baby looks like him, in my opinion, I don't know if I am just paranoid or what but I think it looks like him. He says all babys look the same with squishy faces & that it's impossible for it to be his. I know that he didn't go anywheres near her after I found out last January so I know the very last possible time they could have even done anything was December 31st, which was the last time they were together. Do you think I am just paranoid? It's impossible right, because that would make her pregnant for like almost a year! Agh.