Scoot, I'm sure you will be fine. You won't know until you are doing it....and really, flights are uncomfortable at the best of times, having a belly doesn't really make that much difference...well to me anyway

I am 30 weeks on Sunday and I'm flying tomorrow (a 4 hour flight) with a 2.5 year old.

You'll be fine, if you're uncomfy, just walk around for a little bit, or perhaps log on to the air-line website and try and allocate your seat to one where you can get out easily, or stretch out.