Had Bella from Twilight never conceived a child (Renesmee), the entire trajectory of Breaking Dawn and much of the greater context of the saga would have been considerably different. Among the most dramatic implications are:

1. No Pregnancy = Less Conflict with Volturi

The presence of Renesmee is what ignites the main Volturi battle in Breaking Dawn, when they erroneously believe she is an immortal child—a dangerous, unlawful conception.

Without her, no combined strength of allies or final confrontation, and the Volturi might be an even more distant threat.

2. Edward and Bella's relationship dynamics would change

Most of Breaking Dawn is spent describing Bella's rapid pregnancy and her tortured, ionate metamorphosis.

Without the child, Bella's vampirization could potentially take place in a less confined, less desperate context.

Edward might not feel as guilty and nervous about it as he does during pregnancy.

3. Jacob's Imprinting Wouldn't Happen

Jacob imprint on Renesmee, which ultimately brings him back into the fold with the Cullens.

Without Renesmee, Jacob would never come to accept Bella's decisions or the Cullen family, and this could result in extended hostility or even war.

4. Bella's Vampire Life Would Be Quite Different

A majority of Bella's focus after her transformation revolves around motherhood and adapting to life as a vampire.

Without a child, she would have time to find out more about her new abilities, her romance with Edward, and her place in the Cullen household.

5. The Series Could Have Ended on a Totally Different Note

Renesmee's birth is a resolution and a culmination of some of the novel's ongoing conflicts.

Without her, the climax might be darker, more ambiguous, or more concerned with the dangers of immortality and unlimited love.