The fetal femur length is generally around 3cm at 22 weeks of gestation and grows by 1cm every month thereafter, slowing down after 28 weeks of gestation, reaching 5cm at 28 weeks of gestation, 5.5-6cm at 32 weeks of gestation, 6.5cm at 36 weeks of gestation, and above 7cm at 40 weeks of gestation, which is the roughly normal femur length standard. The specific data and curve can be found on the internet. If the femur length is less than 5% of the corresponding gestational week, it is generally called short femur length, mostly due to genetics, but in some cases it is a fetal chromosomal abnormality, which is very rare.