Biparietal diameter refers to the distance between the parietal bones of the fetus. Clinically, the size of the biparietal diameter is used to determine the fetal growth and development and the normal pregnancy status, and the causes of large biparietal diameter are commonly the following: 1, incorrect calculation of the gestational week: the correct gestational week is projected through ultrasound diagnosis in early pregnancy; 2, excess nutrition: the fetal development is controlled within the normal range by adjusting the diet; 3, fetal malformation: abnormal fetal brain development, resulting in large biparietal diameter. The fetus has a large biparietal diameter. If the fetal biparietal diameter is large, we should pay attention to further check whether the fetus has other abnormalities.