Not PG right now, but I did a lot of planks second and third tri. That is, when I actually did core work...LOL, I sort of skimped on that during my entire pregnancy!
I haven't done much dedicated core work in the latter half of this pregnancy. I do some bench push-ups, and my yoga routine sometimes includes planks, but mostly I've just not really been focusing much on it. I do work my back when I lift weights though.