I think a gift certificate to a spa or salon for a pedicure, massage, etc. are the most awesome gifts for pregnant women. When you're pregnant you kind of have to buy your own clothes because you have to try them on and see what fits or works for you at various stages. I wasn't, but I know a lot of women also find it depressing, so I don't think clothes are a good gift idea. Women love spa treatments, even those who claim they don't... once you get them in there they are in heaven and quickly realize what they've been depriving themselves of!
Jewelry is also a tricky gift for pregnant women because our wrists and fingers often swell and once the baby is born it's sometimes hard to wear earrings and necklaces for quite a while. And I wouldn't buy her gifts for the baby - wait until the baby is here and buy the baby gifts then. Treat the mother now!
Perhaps even tickets to a movie or gift certificates to a book store or a nice dinner date, or fresh flowers every week!