At 24 weeks of gestation, the normal range of fetal biparietal diameter is 5.7-6.6 cm, and the normal range of femur length is 3.7-4.6 cm. Therefore, a fetal biparietal diameter of 5.5 cm is on the small side, and a femur length of 3.9 cm is within the normal range. The size of the biparietal diameter and the length of the femur are generally used to determine whether the development of the fetus is normal during pregnancy. However, due to the influence of genes and environmental factors, the development of each fetus will be different, so the size of the biparietal diameter and the length of the humerus will also have some differences. At present, at 24 weeks of pregnancy, a biparietal diameter of 5.5cm is on the small side, and a femur length of 3.9cm is normal. If the size of the biparietal diameter and femur length of the fetus is not much different from the normal values, regular observation is needed. If the values are obviously abnormal, timely examination is needed to determine whether the fetus has abnormal development. Therefore, if the size of biparietal diameter and femur length is not within the normal range, it is recommended to consult a doctor in time for dynamic monitoring, early detection, early diagnosis and early treatment.