The relationship between fetal biparietal diameter and head circumference is directly proportional; the larger the biparietal diameter, the larger the head circumference; the smaller the biparietal diameter, the smaller the head circumference. Biparietal diameter, also known as fetal head biparietal diameter, is the length of the widest part of the fetal head between the right and left sides. It is often used to observe the child’s development, to determine whether there can be cephalopelvic disproportion and whether the delivery can be successful. Fetal head circumference refers to the maximum length of the circumference around the fetal head, which is generally the longest distance from the root of the nose in the forehead to the occipital tuberosity in the back of the head. It can be used to assess the size of the fetal head to predict the development of the fetus, and it can also be used to help choose the best delivery method when a pregnant woman goes into labor. During pregnancy, doctors need to measure the fetal head circumference by ultrasound to observe the development of the fetus. When measuring the biparietal diameter, the midline of the brain has to be found first, and the vertical distance between the top and bottom is measured according to the midline of the brain, so when the biparietal diameter is bigger, the head circumference will also be bigger. Pregnant women are advised to go to the hospital for regular checkups to make sure that the environment and growth of the fetus are in a normal state.