<b><i>Introduction:</i></b> The prenatal diagnosis of fetal craniosynostosis is challenging, especially in single-suture cases. When sutures are obliterated, sound waves fail to penetrate the cortical bone, creating an evident acoustic shadow on the underlying brain. The objective of this study was to evaluate the yield of the ‘brain shadowing sign' (BSS) as a novel sonographic marker for craniosynostosis. <b><i>Subjects and Methods:</i></b> Patients with an antenatal diagnosis of fetal craniosynostosis (cases) and healthy controls paired for gestational age were enrolled in this retrospective case-control study. Two-dimensional scans were assessed by three examiners for the presence of the BSS and additional fetal findings. <b><i>Results:</i></b> The BSS was clearly depicted in all 24 cases on the first analysis and in 22 cases on the second analysis. No fetus from the control group (n = 48) presented the BSS in any of the analyses. Fifteen cases had isolated craniosynostosis and 9 were syndromic (Apert, Saethre-Chotzen and craniofrontonasal syndromes), which were diagnosed significantly earlier due to additional malformations. <b><i>Discussion:</i></b> The BSS is a novel sonographic marker of craniosynostosis which can be used to increase the diagnostic rate of this rare condition and does not require the use of high-definition three-dimensional transducers to be depicted
