The B ultrasound biparietal diameter is the length of the widest part of the fetal head on both sides according to the ultrasound image, also known as the large transverse head diameter, which is an important indicator of fetal development and can also be used to guide the pregnant woman whether she can have a normal delivery. The fetal biparietal diameter is the distance between the parietal bulge, which is also known as the transverse diameter of the fetal head. There is no standard fetal biparietal diameter, but there is a normal average at a particular gestational week, and if it is out of range, the fetus may need to be evaluated further to see if it is growing too slowly or too fast. In general, the biparietal diameter is about 7.0 cm at 28 weeks of pregnancy, 8.0 cm at 32 weeks of pregnancy, and so on, and after 8 months of pregnancy, an average growth of about 0.2 cm per week is considered normal. A larger biparietal diameter can occur due to better fetal development, better nutrition of the pregnant mother and less exercise. If the fetal biparietal diameter is too large, the presence of hydrocephalus needs to be considered and the pregnancy needs to be terminated if necessary. In addition, if the biparietal diameter is too large, it is not conducive to a normal delivery. If the biparietal diameter is small, the fetus is considered to be underdeveloped. It is recommended to actively supplement nutrition, and at the same time, to conduct regular ultrasound examinations to observe the recovery of the fetus, and if the results of amniocentesis examination show abnormal fetal development, timely induction of labor is required.