Why is Bella so stressed out when she finds out that she is pregnant? She doesnt even know what is gonna happen!!
Bella is worried about her pregnancy because she thinks Carlisle and Edward want to kill her baby. She clearly expresses her fears when she calls Rosalie for help. She knew she would protect the baby because Rosalie wanted a baby.
I think it would be anybody's gut reaction to assume that being impregnated by the undead is going to lead to bad things.