Biparietal diameter and head circumference are positively correlated, the larger the biparietal diameter, the larger the head circumference, both of which are important indicators of fetal head development, but the measurement position and clinical significance are also different: a. Measurement method: 1. The fetal head is the longest part from the front to the back of the head, which is the length of one week from the “root of the nose of the forehead” to the “occipital ridge of the hindbrain”. Clinical significance: 1. Biparietal diameter: the measurement of the fetal biparietal diameter before birth is more accurate, the clinical significance is to determine whether the cephalopelvic symmetry, whether it can complete the normal delivery. If the fetus is too big and the biparietal diameter is too large, making a prediction in advance can reduce the risk of difficult labor. For fetuses with large biparietal diameter, Caesarean section can be adopted directly, which is relatively beneficial to both the fetus and the pregnant woman; 2. Head circumference: this index can measure the head volume of the fetus in the pregnant woman, but also has an important role in measuring the growth and development of the baby after birth. If the head circumference ratio is much smaller than normal fetus, then the possibility of developmental malformation is higher; if the head circumference ratio is much higher than normal fetus, the possibility of hydrocephalus and rickets is higher.