First-trimester fetal neurosonography

First-trimester ultrasound has evolved beyond basic screening to permit structured neurosonographic assessment of the developing fetal brain. Historically, detailed evaluation of intracranial anatomy was deferred to the mid-trimester, based on assumptions regarding developmental maturity and technical limitations. However, advances in imaging technology, improved understanding of early neuroembryology, and refinement of standardized scanning planes have shifted this paradigm. Major structural anomalies that disrupt early cranial morphology, including acrania-exencephaly sequence, holoprosencephaly, and encephalocele, can be reliably detected in the first trimester. Early indirect intracranial markers substantially improve detection of open spina bifida and Chiari II malformation, particularly through assessment of posterior fossa anatomy and quantitative indices such as the brainstem-to-occipital bone ratio. Emerging markers, including choroid plexus-to-ventricle ratios and Doppler evaluation of the pericallosal artery, enhance early identification of ventriculomegaly and agenesis of the corpus callosum. Nonetheless, anomalies involving later-developing structures, such as cortical malformations and vermian abnormalities, may remain undetectable at this stage.