With respect to the assessment of fetal behaviour, there have been numerous reports on the conventional two-dimensional (2-D) ultrasonographic evaluation of fetal movements. However, there are some limitations regarding the assessment of fetal movements using 2-D ultrasound. Because of the 2-D character of real-time scanning, fetal movements outside the scanning plane cannot be displayed on the monitor. The advent of three-dimensional (3-D) ultrasound significantly advanced fetal and placental evaluation in normal and abnormal pregnancies. The introduction of four-dimensional (4-D) ultrasound techniques would allow the assessment of fetal behavioural assessment in three dimensions in real time during pregnancy. We present the latest 4-D sonographic techniques and recent 4-D sonographic studies on fetal behaviour, especially fetal whole-body movements and facial expressions in all three trimesters of pregnancy, and the continuation of behaviour from the fetal to neonatal period. 2-D does not appear capable or comparable to 4-D ultrasound in terms of evaluating early fetal facial expressions. Moreover, we describe the safety of 3-D/4-D ultrasound during pregnancy. This novel technique may assist in the evaluation of fetal behaviour, and offer potential advantages relative to conventional 2-D ultrasound. 4-D ultrasound may be an important modality in future research on fetal neurobehavioural development, although some limitations regarding the assessment of fetal behaviour by means of 4-D ultrasonography still remain unresolved. Copyright © 2009 John Wiley & Sons, Ltd.