Objectives Gestational diabetes mellitus (GDM) carries an increased risk of neurocognitive impairment in offsprings. However, the contribution of maternal hyperglycemia in affecting fetal brain development is not fully elucidated yet. The aim of this study was to evaluate fetal brain and sulci development in pregnancies complicated by GDM. Methods Prospective observational study including 100 singleton pregnancies complicated by GDM and 100 matched controls. All fetuses underwent neurosonography at 29–34 weeks of gestation, including the assessment of the length of the corpus callosum (CC), cerebellar vermis (CV), Sylvian (SF), parieto-occipital (POF) and calcarine fissures (CF). Sub-group analysis according to the specific treatment regimen adopted (n 67 diet vs. 33 insulin therapy) was also performed. Results Fetuses from mothers with GDM under insulin therapy had a smaller CC (35.54 mm) compared to both controls (40 mm; p Conclusions Pregnancies complicated by GDM showed a peculiar pattern of fetal brain growth and cortical development and these changes, which are more evident in those requiring insulin supplementation.