The FL is a mandatory measurement. Apart from knowing about the longitudinal growth of the fetus, measuring the FL has a very important function of excluding dwarfism in the fetus.
By convention, measurment of the FL is considered accurate only when the image shows two blunted ends. The extension to the greater trochanter and the head of femur should not be included. The measurement is also consider inaccurate when the femur image is at an angle of over 30 degrees to the horizontal.
The use of FL in dating is similar to the BPD, and is not superior unless a good plane for the BPD cannot be obtained or that the head has an abnormal shape. There is no evidence to suggest that the length of the femur does not shrink as much as the diameter of the head in case of fetal growth retardation.
Do not take an average of the BPD and FL for dating, because you can always have a fetus with an average size head and a longer or shorter than average lower limb. The measurements should be reported as they are. Do not take an average.
The FL of dwarfs are at least 2-3 SDs below the mean, amounting to 4-5 weeks behind the dates. Abnormality in the shape of the limb will also be present. Remember the lateral surface of the femur is always straight and the medial surface is always curved.
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