I haven't done any preggy photography professionally, but have taken some shots of my wife for posterity. A key factor is how far along they are. Pregnant belly is not going to look nice, much less the mom herself, when the baby is about to pop out. But you don't want to strike too early because she might just look unflatteringly chubby.
Tasteful nudes can be really nice and arty, but definitely aren't for everyone. I like silhouette shots when you want to be very understated. Take this one, overlooking Lake Michigan. She's wearing a sundress and clasping her hands underneath the belly. I could probably do better nowadays with my 5D3 (this was shot with a couple years ago with a Panasonic GH1), but I still like it.