BACKGROUND: Maternal diet has been associated with fetal growth outcomes; however, evidence is scarce on the role of dietary quality. OBJECTIVE: The objective was to assess the effect of diet quality during the first trimester of pregnancy, as measured by the Alternate Healthy Eating Index (AHEI) adapted for pregnancy, on fetal growth. DESIGN: We studied 787 women and their newborns from a Spanish cohort study. Diet quality was assessed by using a modification of the AHEI. Adjusted birth weight, birth length, and head circumference were used as continuous outcomes. We used a customized model to define fetal growth restriction in weight, length, and head circumference. RESULTS: After adjustment of multivariate models, a positive association was observed between diet quality and adjusted birth weight and adjusted birth length. The greatest differences were found between the fourth and first quintiles of the AHEI. Newborns of women in the fourth quintile were on average 126.3 g (95% CI: 38.5, 213.9 g) heavier and 0.47 cm (95% CI: 0.08, 0.86 cm) longer than those in the lowest quintile (P for trend = 0.009 and 0.013, respectively). Women with the highest AHEI scores had a significantly lower risk of delivering a fetal growth-restricted infant for weight (odds ratio: 0.24; 95% CI: 0.10, 0.55; P for trend = 0.001) than did women in the lowest quintile, but this was not the case for fetal growth restriction in length (P for trend = 0.538) or head circumference (P for trend = 0.070). CONCLUSION: A high-quality diet in the first trimester of pregnancy is associated with birth size and the risk of fetal growth restriction.