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To assess the frequency of fetal movement and reproducibility of fetal movement counting in normal singleton pregnancies early in the second trimester using four-dimensional (4D) ultrasound.Twenty-nine singleton pregnancies were studied for 15 min employing 4D ultrasound at 14-16 (19 cases) and 17-19 (10 cases) weeks of gestation. The frequencies of eight fetal movements (head anteflexion, head retroflexion, body rotation, hand to face movement, general movement, isolated arm movement, isolated leg movement and mouthing movement) were evaluated.The most frequent fetal movements were isolated arm movements at 14-16 and 17-19 weeks' gestation. There was a significant difference only in the frequency of mouthing movement between 14-16 and 17-19 weeks' gestation (P < 0.05). All fetal movements showed intra- and inter-class correlation coefficients greater than 0.87, with good intra- and inter-observer agreements.The difference in the frequency of mouthing movement at 14-16 and 17-19 weeks' gestation may be due to increasing fetal swallowing because of the increasing amniotic fluid early in the second trimester of pregnancy. To the best of our knowledge, this is the first study on the reproducibility assessment of fetal movement counting using 4D ultrasound. However, the data and their interpretation in the present study should be taken with some degree of caution because of the small number of subjects studied. Further studies involving a larger sample size are needed to assess the reproducibility of fetal movement counting using 4D ultrasound.