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

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).

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.

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.

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.