OBJECTIVES: To determine the range of positions of the fetal head in which a three-dimensional (3D) volume is acquired for subsequent successful imaging of the corpus callosum.METHODS: We used 3D volumes of the fetal head obtained from singleton pregnancies at 20 to 23 + 6 weeks' gestation. The volumes, which had been acquired with the head in different positions, were then reconstructed with the objective of obtaining a mid-sagittal section of the brain to demonstrate the presence of the corpus callosum.RESULTS: In the reconstructed mid-sagittal sections it was possible to demonstrate the corpus callosum in at least 90% of cases when the 3D volume acquisition plane was (1) mid-sagittal with the angle between the transducer and the direction of the fetal nose ranging from 0 degrees to 179 degrees and from 330 degrees to 359 degrees , (2) oblique around the crown-rump axis with an angle from the mid-sagittal plane of less than 30 degrees , (3) oblique around the anteroposterior axis from the axial plane at the level of the biparietal diameter to the mid-sagittal plane or (4) axial at the level of the biparietal diameter with an angle between the transducer and the midline echo of the brain of 60-119 degrees . In the mid-sagittal sections either the translucent corpus callosum or a comma-shaped echogenic structure was seen depending on whether the plane of volume acquisition was sagittal or axial.CONCLUSIONS: In 3D ultrasound examination the extent to which the corpus callosum can be demonstrated to be present is entirely dependent on the plane of volume acquisition.