Aw, dammit, I posted a reply to you yesterday, but it is not here.
I would say the baby ids definately your fiance's child.
I had my first ultrasound a few weeeks ago ( i''m 13 weeks now, with my second baby)
the technition told me I was 8 weeks and 5 days pregnant- which I was a bit confused about.
Apparently, the count the two weeks before you ovulate as part of the pregnancy- not just for your due date, but for everything including when doing the ultrasound. So when he said I was 8 weeks 5 days, I was actually 6 weeks and 5 days from conception, but it's 8 weeks and five days pregnant all the same. The ultrasound guy explained it really well.
Counting all your dates, that make the 31st about exactly right. And the ultrasounds are really very acurate at that early stage, they realy can tell to the day- not like when the baby starts growing, and they all vary.
I'm sure it's your fiance's baby.
Stop worrying, it will do no good for you or your baby- you're marrying your baby's father, everything will be ok.
Be happy, and congratulations (also, very brave and well done for being honest with your fiance)