1) You are so big
-Really, you're only having one?
-You aren't due until February?
-That's going to be a big baby!
2) My friend/relative/6-degrees person had this awful thing happen to their baby
-I don't want to go into these, it's hard to be polite when listening to stories of otherwise healthy babies that are suddenly still-born, ill, whatever
3) I (only) gained x pounds
-If it's only, x is like 10
-Otherwise, x is something like 90
4) So and so was so happy being pregnant
-Those women are lying, it's all I can say