The biparietal diameter and head circumference of the fetus are different when the position of the fetus is different, when the fetus is in the basin the biparietal diameter and head circumference of the ultrasound will be smaller, so there may be 39 weeks of the biparietal diameter and head circumference are smaller than 38 weeks, but need to be investigated for the existence of intrauterine fetal growth slow. The fetus will not grow too fast in the second trimester, and after the eighth month of pregnancy, the average weekly growth of the biparietal diameter is about 0.2 centimeters is normal. If the biparietal diameter and head circumference measured at 39 weeks is smaller than that measured at 38 weeks, it may be related to the fact that the fetal head has already entered the pelvis, and the head is squeezing and shaping. This can be combined with other data. Femur length and abdominal circumference are not affected by the entry of the fetus into the pelvis, and their measurements at 39 weeks should not be smaller than those at 38 weeks. If combined with other abnormalities, intrauterine growth restriction should be investigated.