Sure, as long as your diet is a reasonable one. Skip the "crash" diets and diet fads, and focus on healthy eating instead. You'll be healthier — and more likely to keep the weight off — if you take it slow and combine your diet with regular exercise. (If you diet without exercise, you'll lose muscle as well as fat. Get active, and you'll lose only fat.)
Many experts advise dropping no more than one pound per week, though some studies show that overweight moms can drop up to two pounds per week (in the short term) without a drop in milk supply.
Hold off on any serious dieting for the first two months of motherhood while you establish your milk supply and settle into life with baby. (Besides, you're likely to shed pounds without effort at first as your body releases water weight and uses fat stores to make milk.) After that, some women continue to drop weight by simply eating to hunger and breastfeeding exclusively. If you need some extra help, go ahead and limit the excess in your diet. (Don't eat less than 1,500 to 1,800 calories per day, and try to keep eating nutritious foods that are rich in calcium, zinc, magnesium, vitamin B6, and folate. And don't attempt liquid diets or weight loss medications while breastfeeding.)