Fetal leg length depends on the value of the femur. The standards for fetal biparietal diameter and femur length change with the week of pregnancy. The biparietal diameter is the length of the widest point between the right and left sides of the fetal head, and the length of the femur is the length of the fetal thighs, which are generally used by doctors to determine the growth and development of the fetus. It is usually measured after 12 weeks, and the exact value will increase with the weeks of pregnancy. At 12 weeks of pregnancy, the biparietal diameter is about 15 mm and the femur length is about 7 mm; at 20 weeks of pregnancy, the biparietal diameter is about 46 mm and the femur length is about 32 mm; at 32 weeks of pregnancy the biparietal diameter is about 80 mm and the femur length is about 62 mm; and at 40 weeks of full-term pregnancy, the biparietal diameter is about 92 mm and the femur length is about 74 mm. The above values are averages, and each fetus has its own individual differences. Therefore, it is important to have regular checkups, and the doctor will have to make a comprehensive judgment on the development of the fetus according to the individual’s situation.