I think a lot of times it could be seen as negative because there's always speculation about if she is or isn't pregnant. Sometimes you might read, "She claims she isn't pregnant but that looks like a bump to me", which could come across as negative-sounding. Also, many celebs are accused of having pregnant bumps when they aren't pregnant...which would definitely be a negative thing.
But if you're talking about obviously pregnant women with huge bellies....while I was pregnant, I was fascinated by what celebs where pregnant at the same time (it always seems that everyone is pregnant while you are) and I can't remember ever seeing anything that seemed negative about pregnant bellies.