Intra- and inter-fetal consistencies and differences in motor activity were studied in 12 healthy nulliparous women during the first half of gestation. Real-time ultrasound observations, lasting 60 min, were performed weekly from 7 to 15 weeks of gestation and at 17 and 19 weeks. The various types of movements were categorized according to a previously developed classification system. Data on general movements during the second half of gestation were carried out on the same group of women and these results are also included in this study. Intra-individually there appeared to be a relative week-to-week consistency in the amount of total motor activity, i.e. the sum of all the movement patterns. There was a slight tendency of intra-fetal consistency when the incidence of general movements during the first half of gestation was compared to that during the second half. The other movement patterns did not show these consistencies, namely, each fetus showed large fluctuations in the week-to-week incidences. Inter-individually there were large differences in the amount of the different types of movements at the various ages, which resulted in wide ranges. The sum of the rank orders which for total activity and general movements, were found weekly, were compared to those which could be expected to occur in a homogeneous population. Two fetuses appeared to be significantly different from the others on mathematical grounds. One consistently showed a high and one a low motor output. Week-to-week stability in the rank orders of the various movement patterns was studied by adding up a sum score of all the weekly changes in ranks. Three fetuses were found to be stable and three showed large fluctuations. The others were in between these extremes. The ranks of the movements from high to low frequency, appeared to be strictly age dependent, whereby all the fetuses followed the same developmental trend. There were no differences between female and male fetuses in any of the movement patterns.