Ask your health care provider how much weight you should gain during pregnancy. A woman of average weight before pregnancy should gain 25 to 35 pounds during pregnancy. Underweight women should gain 28 to 40 pounds during pregnancy.Overweight women may need to gain only 15 to 25 pounds during pregnancy.You may need to gain more or less weight, depending on what your health care provider recommends. woman of average weight before pregnancy should gain 25 to 35 pounds during pregnancy. Underweight women should gain 28 to 40 pounds during pregnancy.