At 28 weeks of gestation, the range of fetal biparietal diameter is 7.24 ± 0.65 cm, and a biparietal diameter of 6.8 cm is within the normal range. Biparietal diameter is the distance between the two parietal bulges of the fetus, and is the maximum diameter of the fetal head. Checking the biparietal diameter of the fetus is an important indicator for assessing the growth and development of the fetus, the size of the fetus and the difficulty of delivery. Fetal size, fetal position and the presence of malformations are among the important factors affecting the difficulty of delivery. A large biparietal diameter in the second trimester indicates that the fetus may be too large for the fetal head to pass through the birth canal. If the fetal biparietal diameter is large, it should be considered whether the pregnant woman has over-nutrition, gestational diabetes mellitus, etc.; small fetal biparietal diameter may be related to fetal growth retardation or malformation.