Apparently, Rob talked with Total Film while promoting Water For Elephants.
While talking with Total Film, Rob tells them that the birthing scene has already been filmed. They also state that Rob says "Yeah, I've done it." "I've chewed it, spat it out"
There is no conformation from Summit so, as always, take this with a grain of salt.
On another note- I can totally see Rob and Kristen filming this and laughing. I cannot wait!