Sonographic examination of the fetal central nervous system: guidelines for performing the ‘basic examination’ and the ‘fetal neurosonogram’

Central nervous system (CNS) malformations are some of the most common of all congenital abnormalities. Neural tube defects are themost frequent CNS malformations and amount to about 1–2 cases per 1000 births. The incidence of intracranial abnormalities with an intact neural tube is uncertain as probably most of these escape detection at birth and only become manifest in later life. Long-term follow-up studies suggest however that the incidence may be as high as one in 100 births1. Ultrasound has been used for nearly 30 years as the main modality to help diagnose fetal CNS anomalies. The scope of these guidelines is to review the technical aspects of an optimized approach to the evaluation of the fetal brain in surveys of fetal anatomy, that will be referred to in this document as a basic examination. Detailed evaluation of the fetal CNS (fetal neurosonogram) is also possible but requires specific expertise and sophisticated ultrasound machines. This type of examination, at times complemented by three-dimensional ultrasound, is indicated in pregnancies at increased risk of CNS anomalies. In recent years fetal magnetic resonance imaging (MRI) has emerged as a promising new technique that may add important information in selected cases and mainly after 20–22 weeks, although its advantage over ultrasound remains debated