To avoid lying on the belly during pregnancy, replace the postures in the spine strengthening series with these alternatives:
Instead of Cobra pose, work in Fish pose.
Lie on your back and keep the arms stretched along side you, palms facing down. Squeeze your legs together, engage your buttocks, and press down into the forearms, palms and elbows, to arch your back up.
Keep using your arm strength to arch the chest high enough to slide to the top of your head, looking to the front of the room.
More advanced versions of fish pose include bringing the hands together into prayer position at the chest, or working with the legs in lotus position.