Incremental Diagnostic Yield of Fetal MRI After Targeted Neurosonography in Fetuses with Agenesis of the Corpus Callosum: A Retrospective Cohort Study

Background/Objectives: This study aimed to assess additional findings reported on fetal magnetic resonance imaging (MRI) after targeted neurosonography in fetuses with agenesis of the corpus callosum (ACC). Methods: This retrospective cohort included 129 singleton pregnancies with complete or partial ACC evaluated at a tertiary referral center between January 2020 and January 2023. All underwent fetal MRI within 7 days of neurosonography. Incremental findings were abnormalities not identified on the preceding examination. Final isolated or non-isolated classification incorporated both modalities. Results: MRI reported at least one incremental intracranial or craniofacial finding in 42/129 fetuses (32.6%). The most frequent findings were cerebellar hypoplasia (15/129, 11.6%), vermian agenesis (11/129, 8.5%), and arachnoid cysts (9/129, 7.0%). Final classification identified 71 isolated and 58 non-isolated cases. Among 77 tested fetuses, 15 had aneuploidy; the frequency was higher in non-isolated cases (14/37 versus 1/40; p < 0.001). Conclusions: MRI provided additional anatomical information after neurosonography in approximately one-third of this referral cohort. These findings support assessment beyond confirmation of ACC, but do not establish diagnostic accuracy, improved parental decisions, or long-term prognostic benefit.