Dear L.,
Well, she is only 18 months old and is dealing with a lot of changes in her life. The changes that are happening inside her little body, and the changes that are happening in her family. You need to be very calm and firm at the same time, in order to give her confidence that things are going to be alright. When she refuses the food, then ask her if she wants to eat anymore. If she says or signals no, take her down from the table, clean her up and give her something to do. Clean off the table and get on with your lives. At the moment she starts to throw food, take her away from the table and put the food away.
Just keep on doing this. Do not let her eat anywhere else but the table. You need to sit with her too while she is eating and talk to her as if you were enjoying the meal too. Keep some extra foods on your plate so that you can see if she is interested in them. Be ready to take her away from the table asap when she starts refusing to eat or throwing - take the food off the floor yourself immediately also. Don't fuss at her. Just be firm and quick.
Just continue doing that and the bad habits will be forgotten. The moment that you try to find somewhere else to feed her is the moment that you start some really strong habits to change. I know one little girl that never did sit with the family to eat, and it caused a looooot of other problems. You don't need that.
This will work out, another thing. Do not discuss this with too many other people, or you will start getting lots of advice - some of which that you do not want to follow. She does not need to be punished in any way.
She will do fine with the new baby, just be sure to give her attention too, and get her a baby doll just before the new one is born. You know, a baby doll with a blanket and bottle. She will love it and take care of it the way that you do the new baby, probably. Nothing is for sure, you realize.
Good luck, C. N.