The respiratory functions of the oropharynx, larynx, and trachea of normal human fetuses in utero were explored by means of realtime, two-dimensional ultrasonography combined with color-flow and spectral Doppler analysis. Coronal and transverse images revealed the maturation of coordinated respiratory activity of these structures in the late second and early third trimesters. Observation of the effects of these movements upon the flow of amniotic fluid in the fetal respiratory tract provides an appreciation of the early role of the upper airway in the modulation of fetal breathing.
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