FWIW my ob always said the only sure way to find out if a baby was too big was to give labour a go!

Don't worry too much about fundus measurements...this time around at 28 weeks I was measuring 2 weeks ahead of dates but that came back into line as bub moved down.

I would be more worried about having that "seed" of an idea planted in your mind that you won't be able to give birth vaginally. Family history is certainly something to take into account but that combined with you having been told that you will be unable to give birth vaginally is going to take a very powerful mindset to overcome (if that is of course what you want).

I would be doing some research on caesarians vs vaginal birth and deciding what you would like "in a perfect world." If you do decide that you want a vaginal birth I would be looking into Calmbirthing/Hypnobirthing and just generally educating and physically preparing yourself as much as possible because you will need that extra mental strength if you start getting nervous. And by the same token, if you decide you want an elective C-section then push for it. It is your birth, after all. Just do your research first.