To write a great fanfiction on this topic, focus on the build - up to the birth. Create tension and foreshadow the tragedy. After Bella's death, explore how it affects the relationships in the story. Don't forget to be descriptive about the pain and sorrow of those left behind.
First, you need to understand the characters well. Make sure Bella's death is in line with her character development. Also, describe the emotions of the other characters vividly, especially Edward's shock and despair.