all i know is that nipple piercings heal quite fast. I had mine done, and about two months later, it was basically fully healed (maybe less than that, my memory is foggy, but definitely not longer than two months). So if you were to get them done now, by the time your boobs became a factor in your pregnancy, you'd be all healed up
But then again, I've never been pregnant, so there might be something I'm missing, I dunno.....