Weight gain during pregnancy by week must be monitored as in the first three trimesters you must gain one to four pounds of weight. This is the sign of healthy pregnancy. You can calculate the weight gain by knowing your BMI which means Body Mass Index. For average weight gain which is 25 to 35 pounds the BMI must be in between range of 18.5 to 24.9. It is recommended to gain one pound per week for optimal growth of the baby.