mspfan haven't been to a music festival while pregnant, but have been with back problems and I honestly think Glasto is the worst for getting around / seating / etc. I know it gets an amazing line up & people love it, but I think it has a really awkward layout compared to lots of others as there are so many choke points (that blasted bridge, all the slippery metal roads btw the tents) and very few of the eateries have seats. (I am comparing to, among others Paleo in Switzerland, PinkPop in NL, even Reading is easier to get around, not that I'd recommend that while pg, bit fighty).
THAT SAID, if you take your own seat (I had a camping chair, made DP carry it) you'll probably be fine. The music's loud enough to hear from space, so being at the back's not so bad and the crowd thins out quickly.
Just realised we're about 20m from the Reading Festival now. Don't think I'll be moshing 3 days before my due date, but am extra glad not booked in at Royal Berks as would have to drive in that way 
Have pretty much finished shopping. Bit of a tight budget (money comes out of parental leave fund as both self-employed) but had some excellent luck. Found a buggy in a charity shop for £10 yesterday :) In good nick & easy to fold - giant though! (fills the boot) Think I may have shopped too early as still tempted to look at so-tiny so-cute things and have no excuse :-/