Molar tooth sign (MTS) is pathognomonic for Joubert syndrome (JS) and related disorders. We report a case in which MTS was suspected in an 'at risk' patient at 16 weeks' gestation by sonography and at 18 weeks by additional ultrasound. It was then confirmed at 20 weeks by MRI. A molecular diagnosis of JS was established after termination of the pregnancy. As genetic testing may not be feasible in JS, the ability to identify MTS sonographically as early as possible is important for affected families.