Women of average weight should gain between 25 and 35 pounds during pregnant, underweight women should gain between 28 and 40 pounds, and overweight women should gain between 15 and 25 pounds, states WebMD. A woman pregnant with twins should gain about 35 to 45 pounds.
During the first three months of pregnancy, women should gain about 1 to 2 pounds and then a pound per week after this period, reports WebMD. Pregnant women should ask their doctors about the amount of weight they should gain. During pregnancy, an average woman requires about 300 additional calories per day. A woman pregnant with twins needs about 3,000 to 3,500 calories daily.