Biparietal diameter is the largest transverse diameter of the fetal head. According to the size of the biparietal diameter, the weight of the fetus is estimated by the corresponding formula. There are many commonly used formulas, such as fetal weight (grams) = 900 × biparietal diameter (cm) – 5200, and then fetal weight (grams) = 1.07 * biparietal diameter * biparietal diameter * biparietal diameter * biparietal diameter + 0.3 * abdominal circumference * abdominal circumference * femur length, length unit are cm. the most commonly used is the first formula, the calculation is relatively simple. Clinical ultrasound examination of the fetus, you can measure the size of the biparietal diameter, biparietal diameter > 8.5cm indicates fetal maturity, biparietal diameter in full-term pregnancy can reach about 9.3cm. By measuring the size of the biparietal diameter, the development of the fetus in the uterine cavity can be observed, and the weight of the fetus can be roughly calculated using the formula. In addition, the transverse pelvic diameter of the mother is measured and compared with the biparietal diameter of the fetus to determine whether there is cephalopelvic disproportion and whether the labor can be normal. During pregnancy checkups, as long as the fetal biparietal diameter, abdominal circumference, femur length and other growth and development indicators are generally within the normal range, you can estimate the fetal weight based on the size of the biparietal diameter, applying the formula, but this method of calculating the weight is not particularly accurate, and there is a margin of error of about 10% up and down.