Since Maxis didn't make a way to select custom maternity clothes, this is pretty much impossible as far as I know. I've included Preg meshes on my versions of every outfit I've converted that had them in the original version, that's really all I can do.
There is a hack now to change maternity clothes, i haven't tried it yet but sure will cos im making preggy clothes for the cpu

You find it here:
http://www.modthesims2.com/showthread.php?t=8259&highlight=Maternity