Background Small for gestational age (SGA) birth weight, a risk factor for infant mortality and delayed child development, is associated with maternal educational attainment. Maternal tobacco smoking during pregnancy could contribute to this association. We aimed to quantify the contribution of maternal smoking during pregnancy to social inequalities in child birth weight for gestational age (GA). Methods Data come from the French nation-wide ELFE cohort study, which included 17 155 singletons. Birth weights for GA were calculated using z-scores. Associations between maternal educational attainment, tobacco smoking during pregnancy and child birth weight for GA were ascertained using mediation analysis. Mediation analyses were also stratified by maternal pre-pregnancy body mass index. Results Low maternal educational attainment was associated with an increased odd of tobacco smoking during pregnancy [adjusted OR (ORa) = 2.58 (95% CI 2.34–2.84)] as well as a decrease in child birth weight for GA [RRa = 0.94 (95% CI 0.91–0.98)]. Tobacco smoking during pregnancy was associated with a decrease in offspring birth weight for GA [RRa = 0.73 (95% CI 0.70–0.76)]. Mediation analysis suggests that 39% of the effect of low maternal educational attainment on offspring birth weight for GA was mediated by smoking during pregnancy. A more important direct effect of maternal educational attainment on child birth weight for GA was observed among underweight women [RRa = 0.82 (95% CI 0.72–0.93)]. Conclusions The relationship between maternal educational attainment and child birth weight for GA is strongly mediated by smoking during pregnancy. Reducing maternal smoking could lessen the occurrence of infant SGA and decrease socioeconomic inequalities in birth weight for GA.