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Objective To explore whether neurosonography can detect differences in cortical development and corpus callosal length in late‐onset small fetuses subclassified into small‐for‐gestational age (SGA) or growth restricted (FGR). Methods This was a prospective cohort study in singleton pregnancies, including normally grown fetuses (birth weight between the 10th and 90th centiles) and late‐onset small fetuses (estimated fetal weight < 10th centile, diagnosed after 32 weeks of gestation and confirmed by birth weight < 10th centile). Small fetuses were subclassified into SGA (birth weight between the 3rd and 9th centiles and normal fetoplacental Doppler) and FGR (birth weight < 3rd centile and/or abnormal cerebroplacental ratio and/or abnormal uterine artery Doppler). Neurosonography was performed at 33 ± 1 weeks of gestation to assess the depth of the insula, Sylvian fissure and parieto‐occipital sulcus in the axial views and corpus callosal length in the midsagittal plane. Measurements were performed offline using Alma Workstation software and were adjusted by biparietal diameter or cephalic index. Linear regression analysis was used to assess the association between the neurosonographic variables and study group, adjusting for confounding factors such as gender, gestational age at neurosonography, nulliparity and pre‐eclampsia. Results In total, 318 fetuses were included, of which 97 were normally grown and 221 were late‐onset small fetuses that were further subdivided into late‐onset SGA (n = 67) or late‐onset FGR (n = 154). Compared to controls, both SGA and FGR cases showed significantly increased insular depth adjusted for biparietal diameter (median (interquartile range), controls 0.329 (0.312–0.342) vs SGA 0.339 (0.321–0.347) vs FGR 0.336 (0.325–0.349); P = 0.006). A linear tendency to reduced Sylvian fissure depth adjusted for biparietal diameter was also observed across the study groups (mean ± SD, controls 0.148 ± 0.021 vs SGA 0.142 ± 0.025 vs FGR 0.139 ± 0.022; P = 0.003). However, differences were significant only between the FGR and control groups. Corpus callosal length adjusted for cephalic index was significantly reduced in FGR cases compared with both controls and SGA cases, while there was no difference between SGA cases and controls (median (interquartile range), controls 0.500 (0.478–0.531) vs SGA 0.502 (0.487–0.526) vs FGR 0.475 (0.447–0.508); P = 0.005). No differences were found in parieto‐occipital sulcus depth between the three study groups. Conclusion Neurosonography seems to be a sensitive tool to detect subtle structural differences in brain development in late‐onset small fetuses. © 2021 International Society of Ultrasound in Obstetrics and Gynecology.