2Kalache KD, Franz M, Chaoui R, et al. Ultrasound measurements of the diameter of the fetal trachea, larynx and pharynx throughout gestation applicability to prenatal diagnosis of obstructive anomalies of the upper respiratory-digestive tract. Prenat Diagn, 1999,19(3):211-218.
3Wolfson VP, Laitman JT. Ultrasound investigation of fetal human upper respiratory anatomy. Anat Rec, 1990,227(3):363-372.
4Richards DS, Farah LA. Sonographic visualization of the fetal upper airway. Ultrasound Obstet Gynecol, 1994,4(1):21-23.