Juliette - I'm doing a post natal exercise class at the hospital physio and the physio says you shouldn't do full sit-ups anyway, they don't exercise the right muscles once you go above a certain level. Just doing a sit up enough to lift your shoulders off the ground and look at one spot on the ceiling with your hands behind your head (look at the spot to protect your neck) uses the muscles in the tummy that have been stretched.

Just a suggestion...