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1. Basic Screening: Three Axial Transverse Planes
Routine second-trimester screening must document three axial planes: transventricular, transthalamic, and transcerebellar. Each plane assesses critical developmental landmarks.
Transventricular Plane: Measures the atrium of the lateral ventricles (normal < 10.0 mm) across the glomus of the choroid plexus, identifying ventriculomegaly and hydrocephalus.
Transthalamic Plane: Evaluates the cavum septi pellucidi (CSP), third ventricle, and biparietal diameter (BPD) / head circumference (HC). Absence of the CSP is a hallmark of agenesis of the corpus callosum or septo-optic dysplasia.
Transcerebellar Plane: Visualizes the symmetrical cerebellar hemispheres, vermis, cisterna magna (normal 2–10 mm), and nuchal fold thickness.
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2. Targeted Neurosonography: Multiplanar Transvaginal Approach
When screening planes reveal anomalies or ventriculomegaly, targeted neurosonography uses high-frequency transvaginal probes through the maternal abdomen or vaginal canal (when vertex) to reconstruct coronal and sagittal views.
Midsagittal Plane: Direct visualization of the corpus callosum in its entirety (rostrum, genu, body, and splenium), the pericallosal artery via color Doppler, and the fastigium and fourth ventricle.
Transfrontal Coronal Plane: Evaluates anterior horns of the lateral ventricles, interhemispheric fissure, and sphenoidal wings.
Transthalamic Coronal Plane: Evaluates the basal ganglia, third ventricle, and corpus callosum cross-section.
Transcerebellar Coronal Plane: Direct coronal assessment of the posterior fossa, cerebellar hemispheres, and tentorium.
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3. Diagnostic Milestones & Pitfalls
Recognizing normal developmental stages prevents misdiagnosis of central nervous system malformations. Key milestones include:
The CSP is consistently visualized between 18 and 37 weeks. Its absence after 18 weeks mandates referral.
The cerebellar vermis completely covers the fourth ventricle by 18 weeks; diagnosing Dandy-Walker malformation before 18 weeks has high false-positive rates.
Ventricular atrium width remains stable at 6–8 mm between 15 and 40 weeks. Any measurement ≥ 10.0 mm requires full diagnostic workup including fetal MRI and TORCH screening.