I had a baby a month ago, and can say that the third trimester was definitely a time to chill out and take it easy. Even walking for more than about 10 minutes gave me round ligament pain and lingering hip stiffness. That being said, working out was the only thing that resolved nausea for me during the first trimester (I went to a level 1 cert when I was 5 weeks and didn't know it...).
A great resource is http://www.crossfitmom.com
Andrea has posted concise summaries of what's appropriate during each trimester. Major things to avoid:
Inversions (ie HSPU, GHD situps)
Slow, heavy lifts (ie DL or anything that requires holding the breath and "bearing down")
Both my delivery and my recovery have been quick, and I credit Crossfit!