I'd avoid crunches altogether to avoid your abdominal muscles separating. That's not to say you can't or shouldn't do abdominal exercises, just more transverse abdominis targeted. I would also avoid high impact to prevent joint injury as ligaments become lax and you have a higher risk of injury. The other thing you might have to avoid is a lot of loaded single leg stuff like a stepper to avoid pelvic pain. In terms of your heart rate, you just need to be sensible and work at a rate where you can still hold a conversation. I am a physio but am speaking mostly from personal experience. I went to the gym right up until 40 weeks. I used mostly machine weights (not at the very end) and mostly upper body. The main thing is listen to your body!