Fetal arm posture was studied longitudinally in 10 uncomplicated pregnancies using real-time ultrasound. Observation were performed at four weekly intervals from 12 to 36 weeks, and at 38 weeks. The percentage of assessments with optimal visualization of elbow, wrist and fingers was 84% at 12 weeks and more than 90% thereafter (range 92-98%). There was a clear developmental trend towards increased flexion. Flexion of the elbow occurred frequently from 12 weeks onwards with an increased incidence at 16 weeks, that in the fingers from 20 weeks onwards and from 28 weeks to term age in the wrist. All three trends were statistically significant. There was considerable intra-individual consistency in terms of the ages at which flexion increased. In fact the increase of flexion occurred one session later in only 6/30 registrations. The combined data of the elbow, wrist and fingers revealed preferential arm posture at 12 weeks with the elbow flexed and the wrist and fingers extended. From 16 to 28 weeks, the predominant posture consisted of elbow flexion, wrist extension and finger flexion and complete flexion thereafter. The development of fetal arm posture does not comply with a proximo-distal trend in that there was first an increase in flexion at the elbow, followed by the fingers and finally the wrist. The implications of our findings for understanding the prenatal development of the central nervous system and subsequent postural adjustments to extrauterine life are discussed.