The fetal weight can be roughly estimated using the fetus’ monthly age, but it can also be calculated by biparietal diameter, abdominal circumference, humeral bone length and femur length under the guidance of ultrasound examination, and there are more formulas for calculating fetal weight, each of which may have a certain error of about 200-300g, and generally the error of not more than 250g with the actual weight can be called predicted to match. 1.Fetal weight (g) = 900 × biparietal diameter – 5200; 2. Fetal weight (g) = 1.07 × biparietal diameter ^3 + 0.3 × abdominal circumference ^2 × femur length; 3. Fetal weight (g) = 747.42 × femur length – 2232.56; 4. Fetal weight (g) = 1049.90 × subcutaneous fat thickness of fetal legs – 2513.51; 5. Fetal weight (g) = 100.97 x head circumference + 110.86 x abdominal circumference + 143.09 x femur length + 331.43 x fetal leg subcutaneous fat thickness – 5168.32; 6. Fetal weight in grams = 260.69 x head circumference – 4973.72; 7. Fetal weight in grams = fundal height x abdominal circumference ± 200; 8. Fetal weight in grams = 171.48 x abdominal circumference – 2686.60. The results calculated by the above formulae are used as a prediction of fetal weight and can only be used as a reference for pregnant women. In general, the difference between the actual weight of the fetus after birth and the predicted value is not too big. The purpose of using ultrasound results and the formula to predict fetal weight is to detect early if there are problems with fetal development and if there may be difficulties in delivering the fetus. In general, the greater the weight, the greater the margin of error is likely to be. However, the normal development of the fetus cannot be determined solely by the predicted weight, but by the doctor’s judgment based on the maternal status, ultrasound findings, fetal position and the values measured in the pelvis.