A cohort study assessed the relationship between dietary intake of vitamin A in 493 healthy mothers before and around conception and adverse birth outcomes associated with environmental toxicant exposures. The cohort, non-smoking women with singleton pregnancies, aged 18-35 years, gave birth at 34-43 weeks of gestation. The women were asked about their diets over one year preceding pregnancy. Measurements of PM2.5 were carried out during the second trimester. Birth outcomes were adjusted for potential confounding factors, including gestational age. Standardized beta regression coefficients confirmed an inverse association between PM2.5 and birth weight (beta = -172.4, p = 0.02), but the effect of vitamin A on birth weight was positive (beta = 176.05, p = 0.05), when the two were adjusted for each other. The negative effect of higher prenatal PM2.5 exposures (above third tertile) on birth weight was significant in women below the third tertile of vitamin A intakes (beta = -185.1, p = 0.00), but not in women with higher intakes (beta = 38.6, p = 0.61). The negative effect of higher PM2.5 exposure on length at birth was significant with lower vitamin A intakes (beta = -1.1, p = 0.00) but not with higher intakes (beta = -0.3, p = 0.56). Prepregnancy nutrition of mothers may modulate the harmful effects of prenatal exposures to pollutants on birth outcomes.