Weight gain depends in part on your body mass index (BMI) before pregnancy. During the first 12 weeks, you may gain only 1 to 5 pounds or not gain weight at all. During the second and third trimesters, you will gain between half a pound and 1 pound per week. The key to healthy weight gain is to increase calorie intake slowly. During the first trimester, no extra calories are usually needed. In the second trimester, you need an extra 340 calories per day, and in the third trimester, you need about 450 extra calories per day.