First-Trimester Fetal Brain

Detection of congenital malformations of the central nervous system is feasible with expert ultrasound performed between 11 and 13 weeks of gestation. Recent advances in prenatal ultrasound have improved detection rate in the first trimester, nearing 100% for certain brain anomalies characterized by gross distortion of brain anatomy, such as alobar holoprosencephaly, acrania–exencephaly–anencephaly sequence, and cephalocele. The routine fetal brain examination includes the transventricular axial and midsagittal planes to evaluate the main cerebral structures and obtain head biometric measurements. If a brain malformation is suspected during this assessment, prompt referral for early neurosonography is warranted to confirm and characterize the anomaly, ascertain prognosis and establish the most appropriate management. Although the optimal approach for early neurosonography remains uncertain, we recommend a multiplanar examination of the fetal brain, including additional axial (such as transthalamic and transcerebellar planes), coronal (like transfrontal, transcaudate, transthalamic, and occipital planes), and midsagittal planes (including the posterior midsagittal plane). This chapter aims to provide an overview of fetal brain ultrasound examination between 11 and 13 weeks of pregnancy.