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Evaluation of the adequacy of reference charts for the accurate identification of fetuses with bone length below the 5th percentile

Abstract 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.