Why does Rosalie carry Bella down stairs?

if your talking about the part i think you are its because Bella is heavily pregnant with Renesmee and Renesmee was draining all the power out of Bella so she couldn't walk. if you haven't read the book and your wondering why would Rosalie be so kind to do that because she hates Bella its because Edward wanted to get rid of Nessie (Renesmee) because she could kill Bella so Rosalie and Bella joined forces to stop Edward killing Nessie.