I work in a law firm, so our dress code is very corporate. I always got by with two pairs of good quality black maternity pants and two stretchy 3/4 length skirts - one black, one charcoal grey. I wore my suit jackets with these as long as I could and then I just found that the skirts with my normal work shoes and pantyhose or the black pants looked fine.

In summer I just wore normal maternity tops with these skirts and pants.

I think that people understand you need to be a bit more casual when you're prg.