I have seen it in a former coworker...the bigger her belly got, the bigger her attitude. And she would admit it and make a conscious effort to control herself; she never went completely berserk.
The most outstanding example is a story that a former nursing instructor of mine told me about herself. She is a normally very even-keeled person, doesn't get flipped out about much at all, very able to handle crises. However, when she was about 36 weeks pregnant, her husband told her that he'd be late for dinner the next night and for whatever reason that struck a nerve with her. She picked up the laundry hamper (full of clothes) and hurled it at his head. Missed him but knocked a big hole in the wall. She isn't the type to make stuff up, so I believe the story.
Personally, I am with CEG on the "mom rage" thing. Mess with me, that's one thing...mess with my son...ooooohhhh. Bad idea.