Fetal Sulcal Maturation Asymmetry: A Sulcus-Specific Analysis of First Appearance Using a Graded MRI Scale

INTRODUCTION Cortical development gives rise to hemispheric asymmetries already observable in utero, exemplified by the early appearance and greater depth of the right superior temporal sulcus (STS). Normative trajectories of sulcal laterality remain poorly defined. PURPOSE To compare, sulcus by sulcus, the gestational age at first appearance of cortical sulci between hemispheres, and to test whether lateralization is sulcus-specific rather than uniformly directional. METHODS Public CRL-2025 fetal MRI data: 149 typically developing fetuses, 20.1-38.0 weeks, one scan each. Fifteen sulci were graded per hemisphere on a four-point scale (Grades I-IV; 0 = absent) adapted from van der Knaap et al. Hemispheres were compared per fetus (Wilcoxon signed-rank, Benjamini- Hochberg); the gestational age at 50% presence was estimated per hemisphere by logistic regression. RESULTS Both STS segments were rightward (posterior 50.3% of fetuses asymmetric, 70/75 rightward; anterior 51.7%, 75/77; q<0.0001) and appeared earlier on the right (2.16 and 1.48 weeks, respectively; q=0.0033). The lateral sulcus was leftward (q=0.021). Precentral, postcentral, cingulate, insular and occipitotemporal sulci were asymmetric in 11-19% of fetuses with no consistent side. CONCLUSION Pace and laterality are sulcus-specific: rightward and early for the STS, leftward for the lateral sulcus, fluctuating in five further sulci. Uniform directional asymmetry does not describe the fetal cortex.