Congratulations!
Ab Rehab can be done during pregnancy, with some modifications.
You will only complete through Phase 3 (omitting phase 4), modifying any other moves that you show signs of intolerance with, and you will be taking full breaths (no half breaths) during the ab rehab moves.
I would, however, HIGHLY suggest implementing the breathing exercises daily throughout your entire pregnancy.
Those are the bread and butter of the program and will help the most for conditioning your body for labor, delivery, and recovery.
Then, after you deliver and are ready to begin exercising again, you can start the program over at Phase 1 for your postpartum recovery.
If you have any concerns, feel free to speak with your doctor.