Objective To assess the association between the maternal diet, according to the degree of processing of food consumption, and birth weight for gestational age and sex. Methods A cross-sectional study with 300 women was conducted from February 2009 to 2011 from a maternity ward in Mesquita, Rio de Janeiro. The outcome was based on sex-specific birth weight for gestational age: small, adequate, or large. A validated food frequency questionnaire was used to estimate the food consumption during the 2nd and 3rd trimesters of pregnancy. The food intake was classified into three groups according to the degree of processing: 1) unprocessed or minimally processed foods and culinary ingredients (oil, fats, salt, and sugar), 2) processed foods, and 3) ultra-processed foods. Descriptive analyses were made to assess the tertiles of the percentage of energy intake of each food group on the outcome and on maternal and infant characteristics. Multinomial logistic regressions were used to test the association of the tertiles of food according to the degree of processing on the outcome (adequate, small, or large birth weight for gestational age and sex). Results The analysis of the food frequency questionnaire from the 300 women indicated that the mean percentage of kcal consumed from unprocessed and minimally processed food and culinary ingredients was 54.0%, while the percentages of energy from processed foods and ultra-processed foods were 2.0% and 44.0%, respectively. The highest tertile of consumption of unprocessed and minimally processed food and culinary ingredients had a protective effect on the prevalence of newborn large for gestational weight in relation to the lowest (OR: 0.13; 95% IC: 0.02 to 0.89; p=0.04). Conclusion High consumption of unprocessed and minimally processed food and culinary ingredients during the last six months of pregnancy might be a protective factor against having a newborn large for gestational weight when compared to mothers with the lowest consumption.