Normal Fetal Long Bone Length from Computed Tomography: Potential Value in the Prenatal Evaluation of Skeletal Dysplasias

Background: The prenatal evaluation of skeletal dysplasias is challenging. Ultrasound (US) has limited sensitivity in evaluating these abnormalities. Low-dose fetal computed tomography (CT) is a helpful adjunct in evaluating the fetus with severe abnormalities, but its use is limited by lack of data on normal fetal bone length. Objective: To obtain CT measurements of fetal long bones and to compare published US normative bone length measures to examine their similarity. Methods: A retrospective search of a radiographic database for gravid patients with CT of the abdomen/pelvis for acute maternal abdomen was performed. Fetal long bone measurements were plotted against gestational age (GA)/standard US measurements. Regression coefficients were calculated (p values <0.05 were considered significant). Bland-Altman plots were constructed to identify any systematic difference between CT and US bone length measures. Results: Twenty-five cases met the inclusion criterion of fetal GA 16-39 weeks. The length of long bones increased with GA, with strong R² values for each measurement. Plots of CT measurements against published 50th percentile sonographic values for age also showed strong and significant correlations, suggesting that values may be used interchangeably, a conclusion further substantiated by the Bland-Altman analyses. Conclusion: A CT database for fetal long bone length is presented which can be used to evaluate the fetus with abnormal bones. Additionally we have established a strong correlation between measured fetal CT long bone values and the published 50th percentile normal values for US, which allows the interpreting radiologist to extrapolate CT measurements to US values in the evaluation of fetal bone length.