You can't "force feed" a fetus. Even if you overindulge in sugar, your own body and insulin may be able to control that elevated sugar and just add fat tissue to you. If the baby is very small, less than 10th percentile for it's age, a placental problem may exist. Unable to give more advice without more specifics.
Most often nothing you can do. How much "underweight"? What weight and what gestational age for that weight?