Aim: To identify reference charts for femoral and humeral lengths enabling appropriate identification of fetuses <5th percentile in one population.
Methods: Two samples of fetuses aged 14-40 weeks were selected from our institution's ultrasonographic database. Regression analysis was used to construct reference charts of femoral and humeral lengths based on the local population (n=901). Femur and humerus length measurements from a second sample (n=1240) were transformed into Z-scores using local and previously published equations. Z-score distributions were used to assess the appropriateness of reference curves for our population. Fetuses aged 18-24 weeks with measurements <5th percentile were identified using each reference equation.
Results: For femoral length, one equation other than the local equation yielded Z-score values within the standard normal distribution (P=0.10), but the histogram was skewed to the right. All Z-score distributions for humeral length fell within the normal distribution (P>0.05), but one was skewed to the right. The numbers of fetuses with femoral and humeral lengths <5th percentile in second-trimester ultrasound examinations varied widely among reference equations used.
Conclusion: Most reference charts assessed underestimated the number of fetuses with long bone lengths <5th percentile in second-trimester ultrasound examinations and were thus unfit for interpretations of biometric data from the study population.