I dont know i'm on your side and it sounds very fishy but my conception was june 6th and due date march 13? Is that odd but seriously you can trap her into a dna test like saying for like organs and something god that sounded dumb I have no clue i'm sorry guess you need to wait it out

Assuming her period was the week before, then her due date would be january 2nd (remember conception can be upto 2 weeks after intercourse)
so if her period had just finished when they had sex (sperm can hang around a while and ovulation can vary) then her due date would be january 7th.
Depending on certain measurements taken the midwife/doctor could have changed her due date, or she may have just calculated it wrong in the first place.

Alternatively she may just be a complete tart who has slept with everything and you shouldn't trust a word she says.

If so get a paternity test (have to wait until baby is born).

Well, the due date doctors give is exactly 38 weeks after the estimated date of conception, which if her conception date was 4-2, then her due date would be december 24th. For a jan 13th due date to have been given by her doctor, her estimated conception date would have had to be april 22nd.