I usually stop situps and hard ab workouts at the 2nd semester. You want those muscles strong, but not inflamed. Light plank holds and exercise ball workouts are good. Plus I find its really easy to oull an ab with all the relaxin and round ligament pain. Let you abs relax drink something with some sugar and see if he gets moving around again.
My doctor did not allow me to do any type of sit ups or bending at my stomach after 20 weeks you need to talk to your doctor before starting any type of exercise.
I agree, my husband does Sports Medicine and he recommends stopping all core work that puts u flat on your back @ 20 weeks, but plank and rotation work is perfect. So keep up the good work but stay off your back