Z-scores of the fetal trachea and bronchial dimension.

Objective To develop Z scores for the trachea and main bronchi in normal fetuses. Methods This was a prospective cross-sectional study in 823 normal singleton fetuses. The tracheal diameter immediately proximal to the bifurcation and the left and right main bronchial diameters were measured from their inner to inner edge in the coronal view. Z scores were created for the trachea and main bronchial diameters using gestational age (GA), femur length (FL), and biparietal diameter (BPD) as independent variables. Results Between gestational weeks 20 and 40, the inner diameters of the trachea, left principal bronchi (LPB), and right principal bronchi (RPB) increased from 1.8 to 4.7 mm, 0.8 to 2.2 mm, and 0.9 to 2.3 mm, respectively. A simple linear regression equation was fitted to model the mean of each diameter. There was however significant heteroscedasticity of the standard deviation (SD) with increasing GA, FL, or BPD. Eventually, the following formula was used to calculate Z scores for the diameters: [measured value - equation for mean]/equation for SD. Conclusion We have developed Z scores for the fetal trachea and main bronchi by applying standard statistical methods. These Z scores may be useful to evaluate the early development of the respiratory system.