To calculate the fetal weight, the fetal biparietal diameter, femoral length and abdominal circumference are determined by ultrasound. The weight of the fetus can be deduced by bringing the measured data into the formula. The formula for calculating fetal weight is as follows: 1. Formula 1: 1.07 x biparietal diameter x biparietal diameter x biparietal diameter + 0.3 x abdominal circumference x abdominal circumference x femur length; (BPD: biparietal diameter AC: abdominal circumference FL: femur length) 2. Formula 2: weight (g) = 900 x biparietal diameter (cm) – 5200 3. Formula 3: weight (g) = uterine height x abdominal circumference + 200 4. Formula 4: preset weight ( EBW) = proportion of biparietal diameter × biparietal diameter (BPD) + proportion of abdominal circumference × abdominal circumference (AC) + proportion of femoral length × femoral length (FL) +/- 15% error value. 5. Formula 5: Weight before 20 weeks of gestation = cube of gestational months × 2 (g), while weight after 20 weeks of gestation = cube of gestational months × 3 (g). The formula listed above is only an estimate, not the actual weight of the fetus after birth. When measuring the fetal head circumference, abdominal circumference and thigh bone length, the fetal position will affect the results. Therefore, the calculated fetal weight is subject to some error, and the actual weight at birth may vary from the predicted weight by 10%-15%. It is only possible to predict the normal development of the fetus based on the prediction and to determine whether to enhance nutrition or to control the diet to prevent the fetus from becoming too large.