The standard fetal biparietal diameter at 39 weeks is (9.21±0.59) cm, but the growth and development of the fetus is affected by various factors such as maternal nutrition, genetic factors and racial differences, and there are individual differences. Therefore, the value is not absolute, as long as it is similar to the standard, it is usually within the normal range. If the difference is large, the following factors should be considered: 1, the fetal biparietal diameter is large: pregnant women’s nutritional supplement excess, will lead to better absorption of fetal nutrition, fetal development is too fast, and then produce the fetal biparietal diameter is large. In addition, the excessive use of sugar and high-calorie foods during pregnancy can lead to gestational diabetes, which can also cause the fetus to have a large biparietal diameter, resulting in the phenomenon of a giant baby. Therefore, pregnant women need to reasonably control the structure of their diet and the amount of diet they consume during pregnancy to avoid oversized fetuses, which can cause a certain burden on the mother and the birth; 2. Small fetal parietal diameter: it is generally common for fetal development to be delayed, and the cause of the delay needs to be further investigated. If it is caused by nutritional deficiency, pregnant women need to strengthen their diet, reasonable meal, to provide more comprehensive nutrition for the fetus; if it is due to disease caused by fetal growth retardation, it is recommended that under the guidance of doctors for further examination, and depending on the situation to decide the termination of pregnancy or not.