Calculating fetal weight using biparietal diameter is usually based on a formula that may require a combination of several indicators. Biparietal diameter refers to the largest transverse diameter of the fetal head, and the weight of the fetus can be estimated based on the biparietal diameter, which is calculated according to some formulas, for example, 900 x biparietal diameter (cm), minus 5,200, or using 1.07 x biparietal diameter x 3 + 0.3 x abdominal circumference x 2 x femur length. Calculating fetal weight based on the biparietal diameter of the fetus, this result will also not be accurate because the biparietal diameter will be affected by whether or not the fetal head is in the pelvis. For example, if the fetal head is highly floated, the biparietal diameter may be measured on the large side. If the fetal head is in the pelvis, the biparietal diameter may be on the small side. In addition to ultrasound to determine the weight of the fetus, the doctor can also estimate the weight of the fetus based on the height of the uterus, abdominal circumference, etc., but there is a certain degree of error.