N.D.
like someone said, it could be your abs split. i knew that had happened after my first pregnancy so when i had my second c-section, i had the dr sew me back up. unfortunately he could only get up to my belly button so my upper abs are still split and i'm working with a physical therapist to put them back. you need them for proper back support. anyway, here are similar exercises that she is having me do http://www.best-abs-exercises.com/stomach-vacuum.html. note that regular sit-ups and crunches can make the gap wider, so you have to be careful.