Even if you do have bub's on your due date, you still have things like bleeding and leaking breasts to think about too. Might not be the best look in your gorgeous dress.
The other thing is, and I hate to say it, but what if something happens and you need a C-sect? You can't lift anything for around 6 weeks after, so I am sure that standing around posing for pics, and doing bridesmade type things might be horrid if that were the case.