its not necessarily a 7-9 month span. They never say how far along she was when she first told Anakin, and she was showing at that point. remember they had been apart for what 'seems like lifetimes' or something. so unless the twins arent really Anakin's then he must have knocked her up when last he was home. which could have been been almost 9 months. Plus you assume that the development rate of a fetus out in a galaxy far, far away is the same here on earth

i think this was all mentioned in one of the other million threads that popped up after ROTS came out, its nice some of those have died off, though they did hold nice ideas/thoughts.. the only thing really left to consider is that it seemed her belly size fluctuated as time went by.

-Darth Simon