Biparietal diameter is the length of the widest part of the fetus between the right and left sides of the head, and is a common indicator of fetal size. In order to observe the growth of the fetus, ultrasound is often used to examine the biparietal diameter of the fetus. There are different reference values for the biparietal diameter at different stages of fetal development. Measuring the biparietal diameter of the fetus and comparing it with these normal reference values can roughly infer the weight of the fetus, and can also determine whether the growth and development of the fetus is normal or not. Biparietal diameter is the largest transverse diameter of the fetal head, which is a commonly used indicator during labor checkups, and to a certain extent, it can determine whether or not the fetus can be delivered smoothly during labor. If a fetus with a large biparietal diameter is delivered in a normal way, it is often difficult to deliver and should be prepared for a cesarean section.