Renesmee Carlie Cullen was born on september 11th, 2006.. If I weere Stephenie, I wouldn't have chosen this petsa because... Well, everyone knows.. But she was born on the 11th, you can find the petsa in the official illustrated guide and Bella's transformation took only 2 days, which is mentioned in The book Breaking Dawn. It only took 2 days because Edward's venom got directly in Bella's heart, and the closer to the heart, the faster the transformation. Bella's transformation was finished on her birthday, september 13th, and renesmee was born on the araw she was transformed, so her borthday has to be the 11th of september, 2006 .
