Ask your health care provider how much weight you should gain. A woman who was average weight before getting pregnant should gain 25 to 35 pounds after becoming pregnant. Underweight women should gain 28 to 40 pounds. And overweight women may need to gain only 15 to 25 pounds during pregnancy. In general, you should gain about 2 to 4 pounds during the first three months you're pregnant and 1 pound a week during the rest of your pregnancy. If you are expecting twins you should gain 35 to 45 pounds during your pregnancy. This would be an average of 1 ½ pounds per week after the usual weight gain in the first three months.