Prenatal head position from 12-38 weeks. I. Developmental aspects.

Fetal head position relative to the fetal body was studied longitudinally in 10 uncomplicated pregnancies by means of real-time ultrasound. Registrations were made at 4 weekly intervals from 12 to 36 weeks, and at 38 weeks. The percentage of assessments with optimal visualization of head position for analysis increased with age. The development of head position involved a change from a midline to a lateralized preference. It was only at 38 weeks that a clear lateralized head position was found with the head being held mainly to the right. The degree of intra- and inter-individual variability was considerable, a finding typical of motor behaviour in the healthy fetus. It is hypothesized that this bias is due to neural maturation. The implications of our findings for the subsequent development of hand preference are discussed.