When you find out you're expecting, everything changes. You are charged with the tremendous responsibility of ensuring that you and your child grow together in good health. There will be vitamins to take, doctor's appointments to make, and of course, you'll have to watch what you eat. You can use a simple formula to estimate your approximate calorie requirements for each trimester, but remember that the quality of food is much more important than keeping an exact calorie count. Focus on whole foods including fruits, vegetables, whole grains, lean protein and low-fat dairy, and skip the ice cream and pickles.
Determine your pre-pregnancy calorie requirement. For women who wish to maintain their current weight, the calorie requirement for a resting state is roughly body weight times 10. For instance, a 140 pound woman would need roughly 1,400 calories if she slept 24/7 just to support basic biological processes such as breathing and digestion.
Multiply your resting energy expenditure by an activity factor to determine your actual calorie requirement. If you are sedentary, spending most of the day sitting, you'd multiply by 1.3. If you exercise for one hour a day, multiply by 1.5 and if you have a physically strenuous job and are very active, multiply by 1.9. A 140 pound woman who is moderately active would need 1,400 calories times 1.5 or 2,100 calories to maintain her weight.
Add calories based on your current trimester. According to the American Pregnancy Association, women in the first trimester should continue to eat normally. In the second and third trimesters, you need an extra 300 calories per day to support fetal growth. The 140 pound woman would need 2,100 calories in the first trimester and 2,400 calories for the remainder of her pregnancy.
Talk to your doctor. Calculations of calorie requirements are only an estimate. You may need more calories if you start the pregnancy underweight, or less if you are overweight. Ask your obstetrician to help you calculate a calorie requirement based on your situation.