Why wasn't Carlisle there to help Bella and Edward, when Bella was about to give birth to Renesmee?

well if you remember in the movie,Carlisle,Esme and Emmett went to go get more blood for Bella and to hunt to be at their strongest for when Bella did give birth,not knowing she would give birth during their absence.Also,Jasper was with Alice,in the movie while Esme,Carlisle and Emmett went off,because Edward heard the wolves coming and he called for Alice and Jasper to help him.Rosalie was taking care of Renesmee.Does that clear it up for you,Ness?I hope it does.

Bella didn't have any blood left and Carlisle wanted to have enough for the delivery of the baby. So he Esme and Emmett went out to get more. Jacob had to distract the wolves while they got away.

Nessie was born premature. Carlisle had gone out hunting, so that he'd be full of blood and less tempted to attack Bella when the baby was due to be born, but then complications (breaking Bella's spine and then the placenta detaching) meant that Nessie had to be delivered early.