cardio is a great way to help you loose the mommy tummy! Also, pilates, the entire process of pilates engages your core, thus making your work more beneficial. As a PP said, plank is a great core workout, but you need to do more than that. You are correct in saying you cant just do crunches, you have to do a overall program, make sure you get 30 minutes of Cardio work (walking/jogging/running) 3-4x a week, then throw in some pilates (take a class, get a tape, or just check you-tube for some video), and make sure you eat healthy and you should see a change, just remember it takes a lot of work, but you can do it!
I also reccomend trying a fitness program, you could always do Baby Boot Camp (http://www.babybootcamp.com) if one is available around you, they make sure you get a full workout which will make sure to help you loose that tummy! (and you and your baby will have fun at the same time :-))
Good Luck! With a little motivation anything is possible!