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OBJECTIVES We sought to investigate the relationship between the usual intake of fatty acids and indices of dietary fat quality in pregnant women and the birth-weight categories of their newborns. METHODS This prospective cohort study was conducted with 734 mother-infant pairs in Brazil. Dietary intake was estimated through 24-h dietary recalls. Secondary data on birth weight, sex of the newborn, and pregnancy duration were obtained. The relationship of fatty acids and indices with birth-weight categories were investigated using logistic regression models adjusted for confounding factors. We considered P values < 0.05 significant. RESULTS The median (interquartile range) maternal age was 27 (23-31) y; 46.2% of the pregnant women had pregestational body mass index ≥ 25 kg/m2, 18.1% had gestational diabetes mellitus, and 11.2% had hypertension. Regarding the newborns, 68 (9.3%) were classified as small for gestational age, 545 (74.2%) as appropriate size for gestational age, and 121 (16.5%) as large for gestational age. In adjusted logistic regression models, a lower chance of being large for gestational age was observed among the children of women classified in the third tertile (versus the first tertile) for intake of polyunsaturated fatty acids (odds ratio [OR], 0.52; 95% confidence interval [CI], 0.31-0.89; P = 0.02), ω-3 fatty acids (OR, 0.48; 95% CI, 0.28-0.80; P = 0.005), and ω-6 fatty acids (OR, 0.56; 95% CI, 0.33-0.96; P = 0.04) and for ratios of polyunsaturated to saturated fatty acids (OR, 0.54; 95% CI, 0.32-0.92; P = 0.03) and hypocholesterolemic to hypercholesterolemic fatty acids (OR, 0.51; 95% CI, 0.30-0.87; P = 0.01). CONCLUSIONS The data suggest that better-quality fat in the maternal diet can reduce the chance of a large-for-gestational-age newborn.