Antenatal imaging of craniosynostosis mainly relies on the demonstration with 2D ultrasound of the abnormal contour of the calvarium and of the loss of hypoechogenicity of the synostotic sutures and on indirect signs of premature closure of the skull sutures; however, isolated craniosynostosis is detected only sporadically at prenatal ultrasound. In this article, we present the first case to our knowledge in which the "brain shadowing sign," a recently described indirect sign of craniosynostosis, noted at 24 weeks in a structurally normal fetus, was the first clue for the diagnosis of isolated bilateral coronal craniosynostosis, which became evident at late gestation.