In order to calculate your weight gain during pregnancy you need to know your BMI before conceiving or at the time of conceiving. BMI is the Body Mass Index which calculates the height and weight relation. The range of BMI which is healthy before pregnancy is 18.5 to 24.9. After pregnancy, pregnant women must reach out to the range of 25 to 35 pounds on average. It is being recommended to gain from one pound to five pounds in the first three months of pregnancy.