The Diencephalon/Falx Ratio: A Novel Mid-Sagittal Marker for First-Trimester Neurosonography.

OBJECTIVES: To establish normative reference ranges for diencephalon diameter, falx cerebri length, and the diencephalon/falx (D/F) ratio between 11 + 6 and 13 + 6 weeks of gestation and to evaluate their relationship with crown-rump length (CRL). METHODS: This prospective observational study included 676 fetuses during routine first-trimester screening; three fetuses were excluded from this study with major cerebral anatomical defects. Using a standardized mid-sagittal plane, the diencephalon (maximum anteroposterior distance) and falx cerebri were measured. The D/F ratio was calculated, and quantile regression was used to generate 5th, 50th, and 95th percentile curves associated with CRL. RESULTS: Diencephalon diameter increased progressively with CRL, while falx cerebri length showed modest growth. The D/F ratio demonstrated a slight inverse relationship with CRL. Two cases of midline defects (agenesis of the corpus callosum) were identified; the D/F ratio was abnormal in one case but remained within normal limits in the second. CONCLUSIONS: Diencephalon and falx measurements are reproducible during the first trimester. While the proposed quantile-based nomograms provide a robust normative framework, the isolated use of the D/F ratio may have limitations in detecting certain midline anomalies. These landmarks should be regarded as adjunctive markers and should be interpreted as part of a comprehensive first-trimester fetal neurosonographic assessment.