Purpose or Case Report: Fetuses who are small for gestational age (SGA) are undernourished and at increased risk of neonatal morbidity. Due to limited nutrient supply, they exhibit a leaner body composition. Fat accretion during late gestation follows a craniocaudal and centripetal pattern, first visible in the cheeks, with the fastest accumulation in the upper limbs. This study aimed to characterize regional variations in fetal fat accretion in SGA compared with appropriate-for-gestational-age (AGA) controls across SGA subgroups by onset (early vs. late) and cerebroplacental ratio (CPR; abnormal vs. normal), and to examine associations with neonatal outcomes.
Methods & Materials: Following IRB approval, SGA pregnancies were prospectively recruited with informed consent, and AGA controls retrospectively identified (consent waived). SGA was defined as estimated fetal weight <10th centile. Cases were classified as early (<32 weeks) or late (≥32 weeks) onset and as normal (≥5th centile) or abnormal (<5th centile) CPR. Adverse neonatal outcomes were recorded.
Fetal MRI was performed at 3T using a whole body T1-weighted 2-point Dixon sequence, generating fat- and water-only images. Subcutaneous fat was segmented using a deep-learning model and manually divided into cheeks, trunk, upper limbs, and lower limbs. Based on subcutaneous fat tissue signal intensities and volume, the fat signal fraction (FSF=fat/[fat+water]) and fat mass (FM=volume×FSF×0.987) were computed globally and regionally; regional FM values were adjusted to global FM (aFM). Linear mixed models were used to compare groups. Logistic regression tested associations with neonatal outcomes.
Results: A total of 64 participants were included: 29 (45.3%) with AGA and 35 (54.6%) with SGA pregnancies; mean gestational age at MRI was 34±1.8 weeks. FSF was significantly lower in SGA across all compartments (P<0.001): cheeks (−9.1%), trunk (−9.3%), upper limbs (−11.9%), and lower limbs (−9.3%). aFM was lower in the upper limbs (P=0.043) but not in other compartments (P≥0.08). No significant FSF or aFM differences were observed between early- and late-onset SGA or between normal and abnormal CPR (P≥0.052). Higher FSF in all subregions was associated with lower neonatal morbidity (P≤0.036), while aFM showed no significant associations (P≥0.084).
Conclusions: SGA fetuses show globally reduced FSF, most pronounced in the upper limbs, and selectively reduced aFM in that region. Regions of rapid third-trimester fat accretion are particularly vulnerable to growth restriction.