| Résumé: | <p>BACKGROUND: We aimed to describe newborn body composition and identify which anthropometric ratio (weight/length; BMI; or ponderal index, PI) best predicts fat mass (FM) and fat-free mass (FFM).</p> <p>METHODS: Air-displacement plethysmography (PEA POD) was used to estimate FM, FFM and body fat percentage (BF%). Associations between FFM, FM and BF% and weight/length, BMI and PI were evaluated in 1,019 newborns using multivariate regression analysis. Charts for FM, FFM and BF% were generated using a prescriptive sub-sample (n=247). Standards for the best predicting anthropometric ratio were calculated utilizing the same population used for the INTERGROWTH-21st Newborn Size Standards (n=20,479).</p> <p>RESULTS: FFM and FM increased consistently during late pregnancy. Differential FM, BF% and FFM patterns were observed for those born preterm (34+0-36+6 weeks’ gestation) and with impaired intrauterine growth. Weight/length by GA was a better predictor of FFM and FM (adjusted R2=0.92 and 0.71, respectively) than BMI or PI, independent of sex, GA and timing of measurement. Results were almost identical when only preterm newborns were studied. We present sex-specific centiles for weight/length ratio for GA. </p> <p>CONCLUSIONS: Weight/length best predicts newborn FFM and FM. There are differential FM, FFM and BF% patterns by sex, GA and size at birth. </p>
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