Biparietal diameter and head circumference are both important markers for determining whether a pregnant woman can deliver normally. Biparietal diameter can also determine whether the skull of the fetus is fully developed and project the weight of the fetus, while head circumference can observe whether the fetus is suffering from hydrocephalus and other diseases. Both biparietal diameter and head circumference are important data for detecting fetal development, but they also have different predictive values. Biparietal diameter refers to the length between the widest right and left sides of the fetal head, it can determine whether the fetal skull is fully developed or not, and in the second trimester you can use the biparietal diameter to project the weight of the fetus. During labor if the length of the biparietal diameter is too large, then the pregnant woman will need to consider a cesarean section. Head circumference is the length of the circumference of the fetus from the point between the eyebrows, through the point of the back of the occiput to the starting point. If the head circumference is too small, then it is necessary to consider the abnormal development of the head of the fetus; if the head circumference is too large, then it is necessary to consider whether the fetus has hydrocephalus. The size of the fetal head circumference is also one of the most important factors in determining whether a woman can have a successful delivery.