Selection of Standards for Sonographic Fetal Femur Length by Use of z-Scores

Abstract

Objectives This study aimed to evaluate which of four established norms should be used for sonographic assessment of fetal femur length (FL).

Study Design Cross-sectional study using pooled data from four maternal–fetal medicine practices. Inclusion criteria were singleton fetus, gestational age (GA) 22^0/7 to 39^6/7 weeks, biometry measured, and fetal cardiac activity present. Four norms of FL were studied: Hadlock et al, the INTERGROWTH-21^st Project (IG-21^st), the World Health Organization Fetal Growth Curves (WHO), and the National Institutes of Child Health and Human Development Fetal Growth Studies, unified standard (NICHD-U). The fit of our FL measurements to each norm was assessed by these criteria: mean z-score close to 0, standard deviation (SD) of z close to 1, Kolmogorov–Smirnov D-statistic close to zero, Youden J-statistic close to 1, approximately 5% of exams <5th percentile, and approximately 5% of exams >95th percentile.

Results In 26,177 ultrasound exams, our FL measurements had the best fit to the WHO standard (mean z-score 0.15, SD of z 1.02, D-statistic <0.01, J-statistic 0.95, 3.4% of exams <5th percentile, 7.0% of exams >95th percentile). The mean of the IG-21^st standard was smaller than the other norms and smaller than our measurements, resulting in underdiagnosis of short FL. The mean of the Hadlock reference was larger than the other norms and larger than our measurements, resulting in overdiagnosis of short FL. The SD of the NICHD-U standard was larger than the other norms and larger than our observations, resulting in underdiagnosis of both short and long FL. Restricting the analysis to a subgroup of 7,144 low-risk patients without risk factors for large- or small-for- GA produced similar results.

Conclusion Of the norms studied, the WHO standard is likely best for diagnosis of abnormal FL.

Key Points

• There are >30 norms for fetal FL.

• It is unknown which norm should be used.

• Our data fit the World Health Organization standard better than the other norms.

Keywords

fetal biometry - fetal Down syndrome - fetal growth restriction - fetal skeletal dysplasia - growth references - growth standards

Publication History

Received: 27 September 2023

Accepted: 19 October 2023

Accepted Manuscript online:
23 October 2023

Article published online:
16 November 2023

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