Fetal occipito-frontal diameter in normal pregnancy: Reappraisal by Altman's model

Background: The purpose of this study was to develop a chart of normal fetal occipito-frontal diameter (OFD) in the Taiwanese population, based on a pure cross-sectional design and without the weaknesses of previously published literature. Materials and Methods: Normal singleton fetuses between 16 and 41 weeks of gestation were enrolled in this study in the Antenatal Ultrasound Unit of the National Cheng Kung University Hospital. Only one scan for each fetus was included. The data were analyzed by polynomial regression analysis, and the equations for the prediction of fetal growth were devised. Altman's model was applied to calculate the standard deviation of each gestational age (GA) before constructing the charts of fetal growth percentiles. Results: A total of 2,048 fetuses with normal growth were included in this cross-sectional study. The best-fitting equation in the prediction of fetal OFD using GA was OFD = 0.74569 x GA - 7.96580 x 10^-3 x GA² - 5.84531 (R = 0.97159, p < 0.0001). The best-fitting equation in the prediction of fetal GA using OFD was GA = 0.44325 x OFD + 0.16763 x OFD² + 11.66546 (R = 0.96411, p < 0.0001). Tables for OFD growth have been generated as a reference for prenatal care. Conclusion: Derived by strictly following the criteria of the study design and appropriate statistical modeling, these fetal OFD values should be suitable for assessing fetal head growth in Taiwan.