I think it's abuit mean to think it can't be a tragedy unless a major character dies!
That's not what we're saying. Everyone pretty much knows what's going to happen in Ep3. And from what we do know, I can't see how it's tragic. And Lucas keeps saying Ep3 is "very very dark".
For me it would only be very very dark if Padme died. What's more dramatic -
Padme gives birth then leaves for Alderaan where she is going to hide.
OR
Padme gives birth and then dies a tragic death.
Plus in AOTC one of the main parts of the plot is Anakin protecting Padme and keeping her alive.
How ironic would that be if he does that but then in Ep3 he turns to the Dark Side which leads to her death?
About the timing thing - I don't beleive what TFN said about Leia and Luke being born at the end. Do you really think Lucas will have the main female character pregnant thoughout the whole film?
If Padme is pregnant at the start of EP3 then she can't really do anything. You won't see her running around a ship fighting bad guys. I think by the start of Episode3 the twins will already be born.