I'm thinking of breaking up my ex and her fiance
Why do you want to break up your ex and her fiance?
It's not because of the guy, but because she's pregnant
I understand that you might still have feelings for your ex, but you shouldn't try and interfere in her current relationship, even if she is pregnant. She has made her decision, and you should respect that and let her be happy.
I really hate the other guy!
It is wrong to mess up someone elses family dynamic. If the relationship is over you should move on with your life.