Birth weight categorization according to gestational age does not reflect percentage body fat in term and preterm newborns

This study was performed to prove the applicability of the small-for-gestational age (SGA), appropriate-for-gestational age (AGA), and large-for-gestational age (LGA) classification depending on birth weight to predict percentage body fat (%BF) measured by dual-energy X-ray absorptiometry (DXA) in term and preterm infants. The data of 159 healthy term and preterm neonates (87 boys and 72 girls) with a gestational age at delivery of 38.4 weeks from two longitudinal studies were analyzed. Anthropometry and body composition data were assessed within the first 10 days after birth. Correlations between anthropometric parameters and fat mass measured by DXA were calculated. Prevalences of observations with low, middle, and high %BF measured by DXA were compared between SGA, AGA, and LGA groups, according to sex and gestational age. In term infants, 42.9% of the newborns with less than 10% body fat were classified to be AGA; 9.9% of all AGA newborns had less than 10% body fat. For the whole group, among the ratios investigated, the weight-length ratio (r=0.82) showed the best correlation to fat mass measured by DXA. The %BF at the time of study was higher in girls (14.75%) than in boys (11.95%). In conclusion, traditional classification based on birth weight centiles does not reflect %BF in term and preterm newborns.