i think maternity shots work best from a 'neutral' angle - that being not so much from the top or bottom but from a 90* angle from her belly - that is, straight on. otherwise things start to distort a little o.O and on pregnant women that can be bad. i mean, it's always fun to mess with angles and such.
also - lots of fabric can be used creatively to flatter and bring focus on the parts that are important, namely, her belly. browse maternity shots on flickr and see what i mean - there's alot of material there. side lighting is good, i don't care for flat, straight on lighting. if you don't have studio lighting then window light or doing a shoot outdoors is always fun. a standard portrait lens is the best.. err on the long side rather than wider unless you're going for that streeeeetched out look on her belly. if you're going for distortion, get really deliberate and get some crazy angles and use a very wide lens to really give that sense of 'oh my gosh, look at her belly!' sort of thing (although she might not like that much lol)... otherwise, for your pretty nice maternity shots follow the tips above
my favorite way to photograph pregnant women is to put them in one of hubby's white button down oxford shirts and a pair of ratty old jeans and have them unbuttoned/zipped so that the belly sticks out and then button the shirt only down to the belly so that it hangs open allowing her tummy to poke out. this is particularly good for outdoorsy type shots.
hope this helps:) if she's okay with it, please share your results!