Dating scans are not that accurate.
Conception dates are based on first day of last period which isn’t when a woman ovulates. That’s on average (and every woman different plus conception can occur several days after ovulation) of 2 weeks later, based on a regular 28 day cycle, which very few women are totally regular because we are not machines.
I had this nonsense from ex!
The only way to know for certain, even if the woman has definitely had sex with more than one man in the same “month” is a dna test.
Do you have any reason other than the dates to think the baby isn’t yours?
My dd supposedly arrived on time based on lmp and scans, yet showed clear signs of being overdue.
“So the exact date they give us isn’t the date then it’s just around that time?” It can be out by weeks either side!
My 2nd pregnancy was supposedly conceived when ex was deployed to another continent (and I definitely wasn’t shagging anyone else) that’s the one caused arguments, dr had to explain menstrual cycles to him (he’s thick)
3rd pregnancy (dd) supposedly conceived according to their dates while in hospital having Gynae surgery!
It’s a “best guess” based on lmp and average cycle length and then measurements at scans, which are also as much an art as a science.
Nobody can say to the exact date when a woman conceived except possibly the woman if eg she’s only had sex once that year!
Stop mistrusting her and support her through HER pregnancy and look forward to becoming a dad.