The standard value of fetal femur length at 21 weeks of pregnancy is 3.64±0.40cm, and pregnant women can combine the data given in the ultrasound report card to see whether the value of their fetus is normal. If in the normal range, then pay attention to do a good job in the later part of the obstetric examination can be, if not in the normal range, may be the femur is long or short of these two cases, mainly caused by the following reasons: 1, measurement error: the fetus in the uterine cavity activity amplitude is large, the body position also change from time to time, different doctors operating differences, will cause the length of the femur measurement data on the error, the error is small, generally do not need to worry about it too much; 2, genetic Factors: if both parents are taller, the fetus may be longer, but if the parents are shorter, the fetus may be shorter; 3, abnormal fetal development: mainly need to be alert to the possibility of intrauterine growth retardation or fetal chromosomal abnormality; 4, calcium abnormalities: if the pregnant woman is deficient in calcium or calcium supplementation is too much, there may be a short or long fetal femur. If it is found for the first time that the fetus has a femur, it is possible that the fetus will have a short femur. If it is the first time to find that the fetus has abnormal femur length, first of all, we should analyze the situation with other indicators such as biparietal diameter, abdominal circumference, etc., and consider reviewing and comparing the results at an interval of 1 week, so as to rule out the cause of measurement error. If there are other abnormalities, further consultation should be made to clarify the cause.