Multiparous mothers have greater umbilical blood flow and thus more efficient transport of pollutants than primiparous mothers. We tested a hypothesis that multiparous mothers are more prone to have an infant with low birth weight (LBW) after prenatal exposure to air pollution. A study was conducted on a representative group of more than 74,000 singleton, live, full-term infants. Birth data were obtained from the birth registry, while pollution data were from an environmental monitoring system (Poland). Multiple comparisons were controlled by the false discovery rate procedure (FDR). After standardization, the harmful effect of carbon monoxide (CO) on the odds ratio (OR) for LBW was seen among the multiparous mothers (OR = 1.28; 95% CI 1.06-1.54), while in primiparous mothers it was nonsignificant. The effect of CO on the OR for LBW differed according to parity, which was confirmed by the test for interaction (FDR-adjusted p = 0.03). The interaction between parity and sulfur dioxide (SO2) was statistically nonsignificant (FDR-adjusted p = 0.08). Multiparous mothers may be more vulnerable to CO than primiparous mothers. Parity may be the modifier of the association between pollutants and the risk of LBW.