The “Falx Flash” sign as an indicator of accurate midsagittal plane alignment for fetal CNS and NT assessment in first-trimester ultrasound

Purpose: Obtaining a true mid-sagittal plane (MSP) and further Fetal Medicine Foundation (FMF) criteria is essential for accurate first-trimester ultrasound screening. The "Falx Flash" sign, a homogeneous echo extending from the thalamus to the cranium, has been proposed as a useful marker. This study investigates the association between the Falx Flash sign and additional key central nervous system structures. Materials and Methods: We conducted a retrospective analysis of 185 first-trimester ultrasound examinations performed between 11+0 and 13+6 weeks of gestation. Images were reviewed for the presence of the Falx Flash sign and fulfillment of the FMF criteria, including visualization of the nasal bone, brainstem, fourth ventricle, cisterna magna, thalamus delineation, and absence of zygomatic bone. Statistical comparisons between the Falx Flash sign-positive and negative groups were made using chi-square or Fisher's exact tests. Results: The Falx Flash sign was present in 70% of cases. Its presence was strongly associated with improved visualization of midline structures: nasal bone (100% vs. 80%, p