Regional differences in fetal fat accretion in small-for-gestational-age fetuses assessed by quantitative magnetic resonance imaging

BACKGROUND: Fetal fat accretion follows a spatiotemporal pattern. Fetuses who are small-for-gestational-age (SGA) demonstrate reduced fat accumulation, but whether specific body regions are disproportionately affected remains unclear. OBJECTIVE: To characterize regional fat differences between SGA and appropriate-for-gestational-age (AGA), assess the modifying effects of SGA-onset timing and cerebroplacental ratio (CPR), and evaluate associations with neonatal morbidity. MATERIALS AND METHODS: SGA pregnancies were prospectively recruited, and AGA controls retrospectively identified. SGA was defined as estimated fetal weight <10th centile and classified as early-onset (< 32 weeks) or late-onset (≥ 32 weeks), with CPR categorized as normal (≥ 5th centile) or abnormal (< 5th centile). Fetal Magnetic resonance imaging was performed at 3-T using T1-weighted two-point Dixon. Subcutaneous fat was segmented and subdivided into cheeks, trunk, upper and lower limbs. Fat signal fraction and fat mass were computed, with regional fat mass adjusted to global fat mass. Linear mixed models compared groups, and univariate logistic regression assessed associations with adverse outcomes. RESULTS: Sixty-four participants (35 SGA, 29 AGA) were included. Fat signal fraction was significantly lower in SGA across all regions (P<0.001). Upper limb adjusted fat mass was reduced in SGA (P=0.043), while other regions showed no differences (P≥0.08). Fat signal fraction and adjusted fat mass did not differ by SGA onset or CPR status. Lower fat signal fraction in all regions was associated with higher morbidity rates, whereas adjusted fat mass was not. CONCLUSION: SGA fetuses exhibit globally reduced lipid content with a disproportionate upper limb fat mass deficit, suggesting selective vulnerability of peripheral fat depots.