Here's the greatest thing you can say to a woman in her third trimester:
"I think you're losing weight! I mean, your belly is getting bigger, but you're shrinking everywhere else. Look at your arms!"
Rationally, I know that this is simply an issue of proportion--the huge belly in front makes everything else seem smaller. Still!
Other good news: my gestational diabetes test was completely normal. And my MIL sent me a fabulous birthday package, a selection of maternity-detracting accessories from the new Coldwater Creek store, plus a gift set of L'Occitane toiletries to tuck in my hospital bag.
I could tell she shopped for this gift personally, which is the real gift to me.