When not pregnant, I always wear these sheer kind of briefs under dresses and skirts to prevent lots of sweating in the hot NYC weather. I don't like them too tight (like a girdle or shaper), it's more for comfort, kind of like wearing bike shorts under dresses.

I previously had ordered a maternity version from Spanx in a size C and I hate it -- it's incredibly constricting -- both for my legs and my baby belly. (Though I think I'll be able to wear them post-baby as a belly constrictor.) Way too much like a girdle.

The Belevation briefs are just right -- I ordered a size larger than recommended (I'm normally a size 10/12 in pants..definitely closer to a 14 now at 36 weeks pregnant). These are snug enough to not be noticeable under a skirt/dress, but loose enough around my legs, waist and belly to be really comfortable. Very stretchy in the best way. Pair this with a real nursing bra - no underwire - and you'll feel so much comfortable when you get to the very-pregnant stage.