4D calculates the baby’s weight by using a formula. The 4D ultrasound measures the values of biparietal diameter, femur length, head circumference and abdominal circumference of the fetus and substitutes them into the weight calculation formula to get the weight. If only the value of the biparietal diameter is known, the fetal weight = 900 x biparietal diameter – 5200, where the biparietal diameter is in centimeters and the weight is in grams. If only the value of the femoral length is known, the fetal weight = 2232 + 747 x femoral length, where the femoral length is in centimeters and the fetal weight is in grams. The value of fetal weight obtained by abdominal circumference calculation is relatively more accurate, and the commonly used formula for calculating fetal weight is Fetal Weight = 1.07 x Biparietal Diameter x Biparietal Diameter x Biparietal Diameter x Biparietal Diameter + 0.3 x Abdominal Circumference x Abdominal Circumference x Femur Length, where the units of Biparietal Diameter, Femur Length and Abdominal Circumference are centimeters, and the unit of fetal weight obtained is grams. It should be noted that the fetal weight calculated by the above method is an estimate, and all the final fetal weights calculated are somewhat different. Specifics should be evaluated under the guidance of a doctor.