Whatever works for you - go for it!! If everything is already planned, I would probably go ahead on schedule. You'll just need to order some of that yummy sparkling grape juice instead of champagne at the reception! A girlfriend and I were both preggers at the same time back in '99 and took a Disney trip - we had a great time waddling around the parks at our own pace.
Regarding your dress - DO NOT GET A MATERNITY GOWN; THEY ARE UGLY!! After working in bridal for ever 5 years, we see this all the time - almost monthly. If your store has agreed to trade in your gown, call ahead and ask the Store Manager or the Seamstress/Alterations Manager to help you look for a replacement gown. A couple of options that work really well:
1. An empire waist gown is a wonderful look for pregnant brides. The bodice will end under your bust and the fullness starts at your ribcage and allows room for baby.
2. Get a two piece gown. Then order the skirt portion with extra length. It sounds weird - but you can pull the skirt up over your waist and up to your bustline (like an empire). Then you can put the bodice over your chest and it will flare out a bit at your waist.
3. Corset styles with the tie back are great too! Then you can cinch it in or let it out as needed.
Finally, with any of these styles, order some extra fabric from the manufacturer. It will cost a couple hundred more, but then your seamstress can make gussets for you. A gusset is a triangular piece that is added into the bodice/waist area to allow for baby room.