The biparietal diameter and head circumference do not determine the size of the head, both are only indicators to determine the size of the fetal head, usually the biparietal diameter and head circumference are proportional, that is, the larger the biparietal diameter, the larger the head circumference, the larger the fetal head. The head circumference, as the name implies, is the length of a circle around the fetal head, which averages 32-34 cm for a full-term fetus. The head circumference is more intuitive than the biparietal diameter. On the one hand, for pregnant women, the biparietal diameter and head circumference can be obtained during pregnancy by ultrasound, and they are generally proportional to the gestational week, so that the fetus can be preliminarily estimated to be in line with the gestational age. Moreover, it can also be combined with abdominal circumference and femur length to roughly estimate the weight of the fetus and predict whether the fetus is huge or not, so as to choose the appropriate mode of delivery, reduce the damage to the birth canal, lower the chance of postpartum bleeding, enable the postpartum recovery as soon as possible and shorten the hospital stay. It is especially valuable for obese or thin pregnant women. In addition, during natural delivery, the biparietal diameter of the fetal head will enter the pelvic inlet plane to complete the articulation gradually, so obtaining the value of the biparietal diameter can also roughly determine whether the fetus has cephalopelvic disproportion, the fetal head is too large and other problems, which has some significance for the choice of natural delivery or cesarean delivery, generally the fetal biparietal diameter of more than 10cm will need to consider cesarean section. For mothers who have already delivered, head circumference is more widely used in child growth and development than biparietal diameter. The average head circumference of a newborn is 32-34 cm at birth and grows gradually with normal growth and development until about 15 years of age, when the head circumference is approximately fixed at 54-58 cm. A small head circumference often indicates that a child may have a brain problem or developmental delay, while a large head circumference or rapid growth should be suspected of brain disease such as hydrocephalus, and prompt medical attention should be sought. Therefore, the size of both the biparietal diameter and the head circumference are related to the size of the fetal head, but at different times, they are applied in different aspects. Both are indicators of healthy growth and development of the fetus and child, so when the test results are abnormal, it is important to pay attention to them and seek medical attention or improve related tests.