The standard value of fetal femur length at 21 weeks of pregnancy is 3.64±0.40cm. Pregnant women can combine the data given in the ultrasound report sheet to see if their fetus’ value is normal. If it is within the normal range, then pay attention to the late maternity checkup later on. If it is not within the normal range, the femur may be long or short, which is mainly caused by the following reasons: 1, measurement error: the fetus is active in the uterine cavity, the body position also changes from time to time, and different doctors operate differently, which can cause the error in the femur length measurement data. If both parents are taller, the fetal femur length may be longer than the average fetus, but if the parents are shorter, the fetal femur length may be shorter; 3, fetal developmental abnormalities: we should be alert to the possibility of intrauterine growth retardation or fetal chromosomal abnormalities; 4, calcium abnormalities: if the pregnant woman is deficient in calcium or has too much calcium supplement, the fetal femur may be short or long. The fetal femur may be short or long. If the fetus is found to have abnormal femur length for the first time, first of all, we should analyze the situation together with other indicators such as biparietal diameter and abdominal circumference, etc. We can consider rechecking at an interval of 1 week to rule out the measurement error. If there are indeed other abnormalities, further consultation should be made to clarify the cause.