The range of the biparietal diameter at 26 weeks is normal between 6 centimeters and 7.4 centimeters. Biparietal diameter is a medical term that refers to the length of the widest part of the fetal head between the right and left sides. It is also the diameter of the head when viewed from the horizontal plane. Because each fetus has different nutrition and developmental rates, it is also necessary to combine the individual femur length and abdominal circumference to make a comprehensive judgment. The biparietal diameter at 26 weeks appears to be on the large or small side. If it is much larger overall, you need to pay attention to controlling your diet and nutrition to avoid over-nutrition. If it is much smaller, there may be malnutrition, and you need to pay attention to micronutrient supplementation and dietary nutrition. During pregnancy, ultrasound is used to detect the biparietal diameter, cranial circumference, abdominal circumference, and femur length, and to find out whether the individual growth and development of the fetus is normal. If there is a significant difference between the individual size of the fetus and the standard range for the corresponding gestational week, it is important to consider the occurrence of fetal growth restriction or macrosomia. If there is a large or small size, or if there are other abnormalities, it is important to follow the physician’s advice and take appropriate measures in conjunction with further testing.