The normal value of fetal femur length at 31 weeks of gestation is 5.5-6.0 cm. The length of the femur becomes longer with increasing weeks of gestation, indicating that the length and weight of the fetus is also increasing. Fetal femur length is highly correlated with maternal condition. Abnormal numerical results are often associated with genetics, maternal malnutrition or overnutrition. If the value of fetal femur length is low, it is most likely that the fetus is thin and small due to genetics and small size of parents, and the femur length is short. If the value of femur length is high, consider heredity, the parents are taller, the fetus femur length will also be long, if there is no other case, no special treatment is needed, most consider the maternal nutrition is excessive, the fetus femur length will also become longer, can follow medical advice to adjust the nutritional intake, excess nutrition is easy to make the fetus bigger or even huge baby, increasing the rate of difficult delivery. In addition, we should also consider whether the fetal femur is long due to fetal malformation. We suggest pregnant women with abnormal fetal femur length values to review and seek medical treatment in a timely manner, as well as to exclude errors in the values due to examination errors.