I have had two children-and two pretty large ones. (10lb4oz, and 9lb4oz premature)
When you have a baby and are trying to get back into pre-baby shape it is a different ballgame than if you are just trying to lose 20-30 pounds but have never had a child.
When you have a baby, your abdominal muscles and the belly skin and everything gets stretched out and weakened. My abs after having each of my children looked literally like a blow up beach ball that you let half of the air out of. The muscles are just competely toast.
Cardio burns a lot of calories, so it will help you get rid of body fat and lose pounds, but to regain the strength and firmness in your abdominal and back regions (both affected by pregnancy) you HAVE to strength train that area-either through traditional strength training methods or through Pilates. The muscle has to be worked back into shape, and there is no way around that.
I mostly do full body traditional strength training-including my back and abs-and it has helped trmendously. I had horrible abs after giving birth, and a lot of back pain from the weakened core muscles-and the strength training that I did/do for my abs and back has been a blessing.