Diet composition may be a modifiable predictor of risk for abnormal glucose tolerance during pregnancy. Prior studies suggest that diets high in total fat, saturated fat, red and processed meats, and with high glycaemic load increase the risk of developing gestational diabetes mellitus (GDM), while polyunsaturated fats, carbohydrates and fibre are protective. The aim of this study was to investigate associations of these and other nutrients and foods, including n-3 fatty acids, trans fats, whole grains and dietary patterns, with risk of GDM. We studied 1733 women with singleton pregnancies enrolled in Project Viva, a prospective pregnancy and birth cohort study in eastern MA. Using multinomial logistic regression, we examined associations of first trimester diet, assessed by validated food frequency questionnaire, with results of glucose tolerance testing at 26-28 weeks of gestation. A total of 91 women developed GDM and 206 women had impaired glucose tolerance (IGT). Pre-pregnancy body mass index (BMI) was a strong predictor for GDM risk (OR 3.44 [95% CI 1.88, 6.31] for pre-pregnancy BMI > or =30 vs. <25 kg/m(2)). After adjustment for confounders, the OR [95% CI] for risk of GDM for total dietary fat was 1.00 [0.96, 1.05], for saturated fat 0.98 [0.88, 1.08], for polyunsaturated fat 1.09 [0.94, 1.26], for trans fat 0.87 [0.51, 1.49], and for carbohydrates 1.00 [0.96, 1.03] per each 1% of total energy. The adjusted OR [95% CI] for risk of GDM for a one standard deviation increase in energy-adjusted glycaemic load (32 units, about two soft drinks) was 0.96 [0.76, 1.22] and for each daily serving of whole grains was 0.90 [0.73, 1.13]. Dietary patterns and intake of red and processed meats were not predictive of glucose tolerance outcome. Estimates for IGT were similar to those for GDM. Intake of n-3 fatty acids was associated with increased GDM risk (OR 1.11 [95% CI 1.02, 1.22] per each 300 mg/day), but not with IGT risk. Except for this finding, perhaps due to chance, these data do not show that nutrient or food intake in early pregnancy is linked to risk of GDM. Nutritional status entering pregnancy, as reflected by pre-pregnancy BMI, is probably more important than pregnancy diet in development of GDM.