OBJECTIVE The purpose of this study was to evaluate the development of fetal lung function during normal pregnancy by analysis of breathing movement-related fetal nasal fluid flow waveforms including its regularity and inspiratory peak velocity using two-dimensional color Doppler and pulse Doppler ultrasonography. METHODS Sixty-eight normal fetuses between 26 and 39 weeks of gestation were examined. Breathing movement-related fetal nasal fluid flow waveforms were recorded. Regularity, frequency, and inspiratory fetal nasal fluid flow peak velocity and transverse lung area were measured. The relationships of these parameters were analyzed. RESULTS A regular pattern of breathing movement-related fetal nasal fluid flow waveforms appeared at 28 weeks of gestation and the appearance of a regular pattern increased to term. In 31 cases there was a regular pattern: frequency of fetal nasal fluid flow decreased and the peak flow velocity increased with advancing gestational age. Positive correlation between inspiratory fetal nasal fluid flow peak velocity and both gestational age and fetal transverse lung area was found. CONCLUSIONS The changes in inspiratory fetal nasal fluid flow peak velocity depend on structural and functional maturation with advancing gestation. The measurement of breathing-movement related fetal nasal fluid flow waveforms using two-dimensional color Doppler ultrasonography appears useful for evaluating fetal lung function.