Femur length (obstetric ultrasound)
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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.
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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- 3. Vintzileos A, Campbell W, Neckles S, Pike C, Nochimson D. The Ultrasound Femur Length as a Predictor of Fetal Length. Obstet Gynecol. 1984;64(6):779-82. - Pubmed