J.D.
PILATES, PILATES, PILATES -- done the RIGHT way. You'll need to wait until you're about 8 weeks postpartum (more if you've had a C-section) for your body to be able to try these exercises correctly....even then, don't be discouraged if you can barely do them. Keep at it and you will see them getting easier each time. The key to pilates (you have to suck your stomach in to your backbone the whole time) is that it really strengthens your core muscles. Sit-ups will not help your condition (I know, I had it too, and now it is gone with the help of pilates) because sit-ups strengthen your vertical abdominal muscles. Pilates will pull those horizontal muscles together to close the gap. You MUST learn to do pilates correctly, though. So go to a class with a decent teacher (someone who is certified in pilates) for a few sessions until you learn how to do it right (I was lucky enough to find a great teacher at my local YMCA)...then you can get videos and do it at home if you want to save money.
Best wishes!
J.