I've done this for a few friends, as a complete amateur just having fun. We actually did them every fortnight so they'd have a record of the pregnancy as it progressed - it's quite a time commitment but very good practice.
I like to pose them against a bare wall with their back to the left of the frame, so their belly sticks out to the right. Maybe it's just me & it might have to do with the way we read from left to right, but I personally think this seems like the 'right' way around when looking at the image.
We start with their shirt down for a few shots & then they lift it up to show the bump - this usually involves some laughter which helps with nice natural expressions. I ask them to move their front elbow forward a bit so the curve of their back is showing, which helps to illustrate the full size of the bump. They usually look a little stiff at first when trying to do this so I ask them to relax their arm while keeping it slightly forward. If their partner or someone else can be there & make jokes or get them to relax in some other way it can help a lot, although this depends on their personality; some people are happier with nobody else in the room.
If you take a series of these with similar background & framing it makes a really nice set at the end of the pregnancy. I give my friends a disk of the images & also include some 'result' shots of the newborn at the end 
There are many more interesting things to do with pregnancy sessions, and looking at the sites of a few pros who specialise in this has given me some great ideas which I may be able to try out one day. Usually though my subjects just want a few snapshots for memories and are not really keen enough to commit to going to a scenic venue or doing more involved posing. Especially if they already have children, they just may not be willing or able to spend very much time on the exercise.
Best of luck with it - let us know how you go!