OBJECTIVE Our study was focused on the evaluation of fluid dynamics to assess the value of gray-scale and color Doppler sonography for evaluating the development of fetal swallow-related movements from early gestation until birth. METHODS We examined 56 fetuses from weeks 15-39 of gestation. Each fetus was examined throughout four distinct periods of gestation: weeks 15-18, 22-25, 30-34, and 37-39. During the examination, seven gray-scale sonography or color Doppler sonography patterns and their prevalence were considered. RESULTS Mandibular and/or labial movements (chi2 = 56.4, p < 0.0001) and their rhythmic activity (chi2 = 41.4, p < 0.0001) were seen on gray-scale sonography in an increasing percentage of fetuses as gestational age increased. Doppler findings showed an increase for nose-mouth flow signals (chi2 = 57.6, p < 0.0001), larynx-esophagus flow signals (chi2 = 13.2, p = 0.0003), and effective swallowing (chi2 = 36.0, p < 0.0001) as gestational age increased. CONCLUSIONS There is a trend in the fetus toward development of increased coordinated movement and more functional nose-mouth flow with increasing gestational age: 32.1% of the 56 fetuses in our series achieved effective swallowing at 37-39 weeks, on the basis of gray-scale and Doppler evaluations. Knowledge of the physiologic mechanism involving swallowing development may allow identification of altered swallow-related movements in fetuses with malformations of the digestive tract or with neurologic disorders.