Don't ever tell a pregnant woman she doesn't look pregnant.
You can tell her she carries it well. You can tell her she doesn't look that far along. You can even tell her that considering she is X months along, she looks great. But even if she is only two months pregnant, don't tell her she doesn't look pregnant.
You especially don't tell her that you can't even tell she is pregnant, implying that this bulge around her mid-section is a part of her everyday physique.
After all, you weren't the one the woke up that morning to discover the four pairs of pants you can fit into are all in the wash and had to stop and have a little cry about that.