The weight of the fetus is calculated based on the data obtained during the ultrasound examination of the pregnant woman, using the fetus’ biparietal diameter, abdominal circumference, and femoral length through a formula to arrive at the weight. Clinically, there are many ways to calculate fetal weight. The most common way to calculate fetal weight is by using the formula for the biparietal diameter, abdominal circumference, and femoral length. Commonly used formulas for calculating fetal weight are: fetal weight in grams = 900 x BPD (biparietal diameter) – 5200; fetal weight in grams = -2232.56 + 747.42 x FL (femoral length); fetal weight in grams = -2686.60 + 171.48 x AC (abdominal circumference). However, the calculated fetal weight is subject to some error, and the heavier the actual weight, the greater the error will be. The doctor’s calculation of fetal weight is one of the important indicators to judge the development of the fetus. If the fetal weight is too low, the fetus may be malnourished and have low resistance; if the fetus is too heavy, the chance of complications during delivery is high and the risk of difficult delivery is also high. Therefore, calculating the fetal weight in the middle of pregnancy can monitor whether the fetus is developing healthily; in the late pregnancy, the fetal weight also determines the mode of delivery for the pregnant woman. Therefore, it is important to pay close attention to the fetal weight during pregnancy and make timely adjustments.