A healthy pregnancy is worth the weight, but bigger isn't better.
Making Healthy Gains
If you are of normal weight, you don’t have to eat more than usual during the first 12 weeks; your fetus’s nutritional needs are minimal. After that, spread your weight gain out over your second and third trimesters. While it will fluctuate from week to week, on average you should gain about a pound per week, depending on your starting weight. Adding 300 extra calories a day after the first 12 weeks will do the trick.
If you are starting out on the heavier side, avoid dieting the first trimester and focus on a well-balanced diet. When the second trimester rolls around, aim for the gradual weight gain recommended by the National Academy of Sciences. As was true in your nonpregnant state, a good way to control your weight is by exercising moderately three to five times a week, doing an activity you enjoy.