bpd is a type of report often seen when pregnant women have fetal ultrasound examinations. It specifically means the fetal biparietal diameter, which is also the length of the longest part between the left and right side of the head, also known as the transverse large diameter of the head. It is often used by obstetricians to observe the development of the fetus and to determine whether there is cephalopelvic disproportion and whether the delivery can be carried out smoothly, which is a very important data in prenatal examination. The bpd value can help the doctor to observe the development of the fetus and is also a reference data. The biparietal diameter length is measured by ultrasound, which is the distance between the two parietal bulges on a standard plane, generally choosing the thalamus plane, measuring the outer extension of the skull, mostly using the distance between the outer edge of the proximal skull to the inner edge of the distal skull, excluding the scalp tissue, because the different thickness of the scalp tissue will produce a certain error. When the biparietal diameter reaches 8.5 cm, it indicates that the fetal head has developed, and the value of biparietal diameter is usually above 9.3 cm when the fetus is full term. The measurement of biparietal diameter is often used to predict the expected date of delivery and is also needed for estimating fetal weight, so it is important for the overall observation of the fetus. When there is a discrepancy between the biparietal diameter and the month of menopause, it is important to pay attention to the fact that it may be caused by various factors such as malformation and maternal malnutrition.