Basically, if the original outfit has pregnancy morphs, so does mine... I haven't removed any morphs from any outfits, but I haven't added any either. Off the top of my head, about half the top separates have them, but not many of the bottoms, if any. It's kinda hit-or-miss, because Maxis had little rhyme or reason on which outfits are pregnancy enabled, and the fact that you can't normally choose your maternity clothing without a hack makes it difficult to work with.
Add the fact that I pretty much never play the game and have never used my stuff on a pregnant sim, I can pretty safely say, I haven't got a clue... but I know they work, 'cause others have told me so.