Quantitative assessment of brain sparing effect in fetal growth restriction: structural evidence from fetal MRI.

OBJECTIVE: To differentiate brain-sparing patterns in fetal growth restriction (FGR) versus small-for-gestational-age (SGA) fetuses using MRI volumetry. METHODS: This retrospective study analyzed whole-body MRI data from 378 fetuses (28-40 weeks' gestation) collected between 2018 and 2025, comprising 308 normal controls, 31 SGA, and 39 FGR cases. Fetal body and brain volumes were segmented using a nnU-Net model. Adjusted linear regression models analyzed growth trajectories of body volume, brain volume, and the brain-to-body volume ratio. Diagnostic performance was evaluated using the area under the curve (AUC). RESULTS: SGA fetuses exhibited an 18.2% lower body growth rate than controls (156.1 vs. 190.9 mL/week) but comparable brain growth, indicating preserved brain-sparing. Conversely, FGR fetuses showed significant reductions in both brain (-18.4%) and body (-15.8%) growth rates. Consequently, the brain-to-body volume ratio decreased rapidly in FGR (-0.005/week vs. -0.002/week in controls), signaling a failure of the brain-sparing mechanism. The ratio model achieved AUCs of 0.989 (Normal vs. FGR), 0.94 (Normal vs. SGA), and 0.883 (SGA vs. FGR). CONCLUSION: SGA is characterized by restricted body growth with relative brain preservation, confirming active brain-sparing. FGR involves significant global restriction and a disproportionately decreasing volume ratio, signifying a breakdown of this mechanism. The brain-to-body volume ratio is a sensitive biomarker for distinguishing these distinct adaptive patterns.