Ultrasound assessment of fetal fibula growth

Background: Fetal fibula growth is interrupted in many congenital short limb syndromes. For clinical application in Taiwan, a normal chart of fetal fibula length (FbL) based on a pure cross-sectional design, and without the prevailing errors in previously published references, was constructed. Materials and Methods: Normal singleton fetuses of gestational age from 17 to 40 weeks were enrolled in this study. Ultrasound examination was performed at the Antenatal Ultrasound Unit of National Cheng Kung University Medical Center in Southern Taiwan. Only one scan for each subject was included. All the subjects were Taiwanese and all the fetuses were followed to delivery to ensure they were normal at birth. The data were analyzed by polynomial regression analysis and searched for the best-fitting equations in predicting fetal fibula growth. Altman's model, using absolute residuals for age- related reference percentiles, was applied to calculate the standard deviation (SD) of each gestational age (GA) before constructing the charts of fetal growth percentiles. Results: A total of 2,184 normal growth fetuses who complied with all the criteria were included in this cross-sectional study. The best-fitting equation in predicting fetal FbL using GA is FbL = -2.56399 + 0.30543 x GA - 2.27871 x 10^-3 x GA² (R = 0.93604, p < 0.0001). The best- fitting equation in predicting fetal GA using FbL is GA = 37.58008 - 16.51825 x FbL + 4.78560 x FbL² - 0.33764 x FbL³ (R = 0.94732, p < 0.0001). Tables for FbL growth percentile charts are calculated as reference for prenatal care. Conclusion: With strict criteria of study design and appropriate statistical modeling, these fetal FbL values are more accurate for assessing the growth of fetal fibula bone in Taiwan. Clinical use of these values in detecting fetal short limb syndromes associated with abnormal fibula growth is warranted. However, the authors emphasize that the predicted values should never be utilized alone in decision-making. In clinical practice the predicted values should always be considered together with the SD.