During the first trimester, it is important that you make sure to limit any excess calorie intake. After the first 12 weeks pass, you can add an additional 300 calories per day in addition to your regular calorie intake.
If you are of normal weight, you should expect to gain 25 to 35 pounds during your pregnancy. You should limit your weight gain to no more than 5-10 pounds in the first 20 weeks, and then following that a pound per week.