Femur Length (FL)
Femur length measures the longest bone in the body and reflects the longitudinal growth of the fetus. Its usefulness is similar to the biparietal diameter (BPD, which is the diameter between the 2 sides of the head.
Femur length increases from about 1.5 cm at 14 weeks to about 7.8 cm at term. (see chart) Similar to the BPD, dating using the femur length should be done as early as possible.
This femur length calculator creates a graph to show you whether the femur length on the ultrasound is within normal for the weeks gestation.