Handedness is the most prominent manifestation of behavioural lateralization in the human population. However, little is known about its ontogeny. This study examined the prenatal development of behavioural lateralization by observing, longitudinally, the occurrence of left- and right-arm movements in 17 human fetuses at 3-week intervals from 12 to 27 weeks gestational age. Fetuses exhibited significantly more right-arm movements than left-arm movements at every gestational age. Throughout all periods of observation, 83.3% of fetuses showed more right-arm than left-arm movements. The number of arm movements observed peaked at 15 to 18 weeks' gestation and declined rapidly in mid-gestation. The occurrence of laterality in early gestation indicates that asymmetric behaviour is probably under muscular or spinal, rather than cortical, control and points to a 'genetic' origin for such behaviour. The presence of lateralized motor behaviour in early gestation suggests it may have a potential causative role for subsequent lateralized behaviour and asymmetric brain development.