I'm going to tell my wife no about having another baby
Why do you not want to have another baby?
I want to give her a divorce. I'm kind of tired of her.
Be open, honest, and direct with her about your decision. Let her know if this is final for you or if you are open to having future discussions about it.
I'm afraid if I ask, she might want to keep the baby.
If she is already pregnant, you need to have the discussion now. You need to be open and honest so you both can make a good decision.