Why is the femur length used as a standard to judge fetal development?

Patient's question:

No

Doctor's answer:

The length of the femur is a commonly used indicator for doctors to observe the development of the fetus during pregnancy when performing ultrasound (or color Doppler ultrasound) examinations. The length of the femur is also one of the important data for calculating the fetal weight during pregnancy. As the gestational age increases, the value of the femur length gradually increases, indicating that both the length and weight of the fetus are gradually increasing.