Bella turns into a vampire on her 19th birthday, September 13, 2006, in the timeline of the Twilight saga. This transformation occurs just after she marries Edward Cullen on August 13, 2006, and gives birth to their daughter less than a month later on September 11, 2006. Edward injects his venom into Bella to save her life after the difficult birth, initiating her transformation. She awakens as a vampire on her birthday, September 13, 2006.
The transformation process is intense and painful, involving days of slipping in and out of consciousness, during which Bella acquires superhuman qualities. After awakening as a vampire, she gains an enhanced mental shield ability and must be carefully integrated into the vampire world.
In summary, Bella officially becomes a vampire on her 19th birthday, September 13, 2006, following her wedding and the birth of her daughter.
