My rule has always been to keep them pretty much at the activity level they're used to after the first trimester. Before then, and in the absence of any potential difficulties, they can train as though they were not pregnant, i.e. start a new program, lift heavy, condition hard. After this 3 months, it's probably unwise to introduce significant intensity or volume to her if she's not adapted to it already. A gal that's been training heavy for years will probably be fine training heavy up until 8+ months if her size does not significantly change her technique for squats and pulls. I've seen women PR the squat at 8+ months. This level of strength and conditioning also seems to shorten labor. Our own Sarah Kim was pushing the prowler in the heat up until 7 months.