Short Answer – It depends how much you weigh when you get pregnant.

In general, 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.

The American Congress of Obstetricians and Gynecologists (ACOG) publishes weight gain guidelines based on BMI (Body Mass Index) ranges.  Your BMI is calculated based off your height and weight. BMI Calculator

 ACOG Guidelines:

Pre-Pregnancy Weight CategoryBMIRecommended Total Weight Gain 
  UnderweightLess than 18.5  28-40 Pounds
  Normal18.5 – 24.9  25-35 Pounds
  Overweight25.0 – 29.9  15-25 Pounds
  Obese30 and Greater  11-20Pounds

Want to know what’s coming in the next few months? Check out this bestseller, What To Expect When You’re Expecting. A great resource for expectant moms, especially first-time moms.