These are gifts for her, not for her future child.
+New Look has a great, basic, "welcome to your pregnant body", starter set. You can't have too many basics. Just as a guide, whatever size they were before they became pregnant: that is the size they are in their maternity clothes. Just imagine them without the bubble; if they look like a medium without their baby bump, then they are a medium in maternity clothes.
I don't know if every pregnant woman will want these pajama's by +Topshop, I just know that I want them. I want them bad; can't imagine I am the only one. Lounge wear is probably the best investment for pregnancy.
Stripe Pajamas, £38
If she's 6 months or more chances are she can't see her feet. Help her keep them cozy warm with these slippers from +schuh. Bonus: the rubber soles are non slip.
Slippers are a good choice but you could also take her to get a pedicure together, since she probably can't reach her feet either.