BACKGROUND Inflammation during pregnancy has been linked to adverse maternal and infant outcomes. There is limited information available on the contribution of maternal diet to systemic inflammation and pregnancy health. OBJECTIVE The objective of this study was to examine associations of maternal prenatal dietary inflammatory index (DII), a composite measure of the inflammatory potential of diet, with markers of maternal systemic inflammation and pregnancy outcomes. METHODS We studied 1808 mother-child pairs from Project Viva, a pre-birth cohort study in Massachusetts. We calculated the DII from first- and second-trimester food-frequency questionnaires by standardizing the dietary intakes of participants to global means, which were multiplied by the inflammatory effect score and summed. We examined associations of DII with maternal plasma C-reactive protein and white blood cell count in the second trimester and the following perinatal outcomes: gestational diabetes, preeclampsia, length of gestation, fetal growth, mode of delivery, and duration of breastfeeding. We used multivariable linear and logistic regression models to analyze the strength of these associations. RESULTS Maternal age was (mean ± SD) 32.2 ± 5.0 y, prepregnancy body mass index (BMI; in kg/m(2)) was 24.9 ± 5.2, and DII was -2.56 ± 1.42 units with a range of -5.4 to 3.7. DII was positively correlated with prepregnancy BMI (Pearson'sr= 0.13,P< 0.0001). Higher DII scores, reflecting more proinflammatory dietary potential, were associated with higher second-trimester plasma CRP (β: 0.08 mg/L per 1-unit increase in maternal DII; 95% CI: 0.02, 0.14) and lower birth weight for gestational agezscore in infants born to obese mothers (β: -0.10zscore per 1-unit increase in maternal DII; 95% CI: -0.18, -0.02). Higher DII scores were associated with lower odds of breastfeeding for at least 1 mo (OR = 0.85; 95% CI: 0.74, 0.98). CONCLUSION A proinflammatory diet during pregnancy is associated with maternal systemic inflammation and may be associated with impaired fetal growth and breastfeeding failure.