Fetal biparietal diameter is the widest length between the right and left sides of the fetal head. Fetal biparietal diameter is the widest length between the right and left sides of the fetal head measured by ultrasound. The standard of fetal biparietal diameter varies in different gestational weeks, which can be used to predict the expected date of delivery, project the weight of the fetus, and assess whether it can be delivered naturally. In general, the biparietal diameter of the fetus should reach 9.3 centimeters or above at full term. Fetal biparietal diameter can be used to determine the size of fetal development, in the process of labor and delivery of the fetal head through the birth canal is the largest diameter of the biparietal diameter, so if the biparietal diameter is larger than the transverse diameter of the pelvic outlet, the cephalo-pelvic disproportion, the natural delivery of the difficulty, if necessary, need to carry out a cesarean section. Fetal biparietal diameter is one of the indicators that need to be evaluated regularly in the second trimester, and it is recommended to follow the doctor’s instructions and conduct regular labor checkups to monitor the situation.