In a longitudinal study of 12 healthy nulliparous women the developmental course of specific fetal movement patterns was investigated in the first half of gestation, using real-time ultrasound. The rate of occurrence of all movement patterns emerging during this period is presented. There were large differences in incidence between the various movements. In most of them a developmental trend was found, either a gradual increase in incidence as the fetus grew older (breathing movements, head rotations, jaw openings, sucking and swallowings), an increase in incidence until a plateau was reached (general movements, isolated arm movements), or an increase in incidence followed by a decrease (startles, hiccups, hand/face contacts, retroflexions of the head). In a few infrequently occurring movements no developmental trends could be observed (isolated leg movements, anteflexions of the head, yawns, stretches) whereas in some of the more frequently occurring movements a lower limit of the normal range could be defined. A few movement patterns were found to be generated at a more or less regular interval: hiccups occurred with a preference interval of one to three seconds and isolated arm movements with an interval of about 1 s. Breathing movements showed a clear developmental shift in preference interval. Between 10 and 19 weeks this changed from 2 to 3 s to less than 1 s. The majority of movements, however, seemed to occur at no regular interval or the regularity was missed because the burst length was too short. Quantification of fetal quiescence showed that in between 8 and 19 weeks total absence of movements never lasted longer than 13 min.