Losing weight while pregnant

I am possibly pregnant with #3. I'm almost 21. My body has never "snapped back" after pregnancy. I gained 60 pounds with each. I have a small bone structure and weight almost 200lbs at 5 feet 5 inches. My Ob and dietitian has suggested a safe diet and routine to help keep my weight down during my current pregnancy. She has also said something about weight loss being possible during pregnancy. Has anyone ever successfully lost weight. (A reasonable amount) during pregnancy?

Without complications.