My wife and I read a lot about it and try to keep 3 things in check: heart rate, body temp, and physical danger to the baby.
Chris touched on the first 2. No WODs in the midafternoon garage, and a HR monitor is a great idea.
As for the physical danger to the baby... no handstand pushups, full squat snatches, box jumps, burpees, etc. CrossFit Mom has subs for all these.