The subplate is a transient structure essential for normal development of the cortex. We used magnetic resonance imaging of the fetal brain to assess cortical subplate evolution between 20 and 35 weeks gestation. Two-dimensional measures of diameter were obtained for the cortex, subplate and fetal white matter. The subplate was originally seen as a continuous band at early gestations measuring up to 4.5 mm. It became magnetic resonance invisible from approximately 28 weeks initially from the depths of the sulci and then from the tops of the gyri. The disappearance of the subplate was regional, involuting most rapidly in the parietal lobe and remaining prominent in the anterior temporal lobe up to 35 weeks.