Mild Ventriculomegaly, Ventricular Asymmetry, and Posterior Fossa Variants: A Narrative Review of Borderline Fetal Brain Findings

Recent advances in fetal neurosonography have markedly improved the detection of subtle central nervous system findings that were previously identified less frequently. Mild ventriculomegaly (MV), non-dilated ventricular asymmetry (NDVA), mega cisterna magna (MCM), and Blake’s pouch cyst (BPC) are increasingly encountered during routine prenatal ultrasound and, in the absence of associated genetic or infectious abnormalities, are commonly regarded as borderline conditions or anatomical variants. These findings frequently pose challenges in prognostic evaluation and in the counseling of expectant parents. The objective of this review is to summarize current evidence on the detection, characterization, and clinical management of these four entities and to provide a practical framework for evidence-based parental counseling. This narrative review examines the diagnostic approach to MV, NDVA, MCM, and BPC using both ultrasound and fetal magnetic resonance imaging (MRI). A uniform analytic framework is applied to each entity, addressing in turn etiology, sonographic assessment, the complementary role of MRI, genetic evaluation, and prognosis with counseling considerations. Representative ultrasound and fetal MRI images from a tertiary referral center are used for illustrative purposes. Across all four entities, fetal MRI provides complementary information in selected cases, while genetic testing and serological screening may help exclude chromosomal or infectious etiologies. Neurodevelopmental outcomes are broadly favorable when the finding is truly isolated, but the prognostic literature is limited by small samples, heterogeneous definitions of “isolated,” short follow-up, and unstandardized neurodevelopmental assessments. Common themes across these entities are synthesized into a staged framework intended to support evidence-based parental counseling regarding the expected neurodevelopmental outcomes of these fetuses.