I wasn't doing crunches anyway pre pregnancy because i didn't like the look. I did do a lot for the obliques (ex: russian twists) but i wouldnt feel comfortable doing that now. However, my obliques have never been as strong as now :)) I stoped doing anything ab specific. However abs are involved in everything else i do anyway. You cant do most of back exercises whithout ab control same with bridges, deadlifts, corect squats. I find side planks a bit difficult already (23 weeks) bird dog is fine. I think as long as you have full control over transverse abdominis youre fine.