Never ask a woman if she is pregnant right?  The explanation given to me is that if it turns out she is not pregnant you are in big trouble.  But, what if I keep quiet and she really is pregnant.  Then she’s thinking “he doesn’t think I am pregnant.  That means he thinks I am actually fat in real life.  Bastard.”  So I am not sure I agree with the conventional wisdom here.

Maybe you are just being cautioned against equivocation.  If you ask then you don’t know and whatever the answer is, your uncertainty reveals that you considered it a possibility that she’s fat.  Under this theory the right strategy is to use your best judgement and just come out and pronounce it with no hesitation.