It is not safe to lose weight during pregnancy. However, if you stay on a healthy diet – eating about 300 calories extra for the baby, and avoiding any junk completely – you will probably naturally gain less than a thinner woman, and redistribute the weight you have a bit, so some can go to baby. Also, exercise is very healthy during pregnancy, and that is sure to help. However, don’t go on any extreme diets. Check with your doctor about what you want to do. But as long as you are eating healthy and listening to your appetite (i.e. don’t starve yourself if you feel hungry), it should be safe.
Afraid I’m not in the same boat as you … I was on the light side before and have gained over 25 lbs at 7 1/2 months. But I haven’t been restricting myself at all, and I admit I’m eating a lot of junk. I’m kind of okay with being a little heavier after the baby is born than I was before.
My advice is to try to lose the weight after the baby is born. Are you planning on nursing? My mom always lost weight without any effort when she was nursing. The pounds go onto the baby and stay off of you!