Femur length (obstetric ultrasound)

Last revised by Jeremy Jones on 6 Apr 2023

Fetal femur length (FL) is a basic biometric parameter to assess fetal size. Femur length, together with biparietal diameter, head circumference, and abdominal circumference, are computed to produce an estimate of fetal weight. In the second trimester, this may be extrapolated to an estimate of gestational age and an estimated due date (EDD).

Femur length can be used to calculate fetal length, another predictor of fetal growth, with the following equation 3

Fetal length in centimeters = 6.18 + 0.59 x femur length in millimeters.

Measurement criteria:

  • measure with the transducer aligned along the long axis of bone - perpendicular to the shaft

  • measure where the ends of the bones are blunt and not pointed.

  • measure from the blunt end of the proximal femoral epiphysis to the blunt end of the distal femoral epiphysis

  • measure post 32-34 weeks when the distal femoral epiphysis can be seen (although not measured)

  • never measure when there is excessive bowing, demineralization or shortened length

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Cases and figures

  • Case 1: normal FL
    Drag here to reorder.
  • Case 2: second trimester scan
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