BPD in ultrasound is the abbreviation of fetal biparietal diameter, and BPD is a term commonly used in obstetric ultrasound fetal examination. BPD is the distance between the parietal process on both sides, and is the largest transverse diameter of the fetus. BPD is commonly used in clinical practice to estimate gestational age and growth, and the size of BPD is also an important indicator for choosing the mode of delivery. The measurement is done by taking a transverse section of the head, the thalamus plane, and the measurement is done from the midpoint of the hyperechoic skull plate to the contralateral midpoint, or from the lateral edge to the medial edge, or from the medial edge to the lateral edge. The measurement of biparietal diameter can effectively reflect the size of the fetal head, as well as the overall growth and development of the fetus in combination with the measurement of femur length, humerus length, head circumference and abdominal circumference, which provides important clinical significance for estimating the gestational week, predicting fetal weight and choosing the mode of delivery.