First-trimester neurosonography of the fetal brain: when visualization outpaces biological interpretability

Over the past decade, first-trimester neurosonography has undergone a remarkable transformation. Advances in ultrasound technology, image resolution, transvaginal imaging, three-dimensional ultrasound, image post-processing, and operator expertise have considerably expanded assessment of the developing fetal brain between 11 and 14 weeks' gestation. Cerebral structures that were previously inaccessible can now be consistently visualized, extending the possibilities for earlier prenatal diagnosis (1). As imaging performance continues to improve, however, a new challenge has emerged. At this gestational age transient embryological structures coexist with definitive anatomy (2), and many cerebral structures can now be identified before they have reached sufficient anatomical maturity to allow reliable diagnostic interpretation. The question is therefore no longer simply how early the fetal brain can be imaged, but when developmental anatomy allows a structure that can be visualized to also be reliably interpreted, and when that interpretation becomes clinically actionable. This concept is illustrated throughout this review through three representative examples: first-trimester ventriculomegaly, the developing future cavum septi pellucidi, and cystic appearances of the posterior fossa. Although involving different anatomical regions, these entities share a common principle: the interpretation of early neurosonographic findings is intrinsically dependent on developmental stage and structural maturation. Rather than reviewing these entities as isolated diagnostic challenges, this article proposes a developmental perspective on first-trimester neurosonography. By integrating current imaging capabilities with the principles of embryological brain development, we explore how developmental biology increasingly defines the opportunities and the limits of early fetal brain assessment., and we propose practical points to guide assessment, follow-up, and counselling (Table 1).