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Femur length (obstetric ultrasound)

Last revised by Dr Jeremy Jones on 20 Sep 2021

Fetal femur length (FL) is one of the basic biometric parameters used 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.

See also

  • automatic online fetal length calculator based on femur length from www.perinatology.com

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Cases and figures

  • Case 1: normal FL
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  • Case 2: second trimester scan
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