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

Patient's question:

No

Doctor's answer:

Femur length is a commonly used indicator for doctors to observe fetal development during pregnancy when conducting ultrasound (or color Doppler ultrasound) examinations for pregnant women. It is also one of the key data for estimating fetal weight during pregnancy. As the gestational age increases, the femur length value gradually increases, indicating that both the length and weight of the fetus are gradually growing.