Although low socio-economic status has consistently been associated with lower birthweight, little is known about the factors whereby socio-economic disadvantage influences birthweight. We therefore examined explanatory mechanisms that may underlie the association between the educational level of pregnant women, as an indicator of socio-economic status, and birthweight. The study was embedded within a population-based cohort study in the Netherlands. Information on maternal education, offspring's birthweight and several determinants of birthweight was available for 3546 pregnant women of Dutch origin. Infants of the lowest educated women had a statistically significantly lower birthweight than infants of the highest educated women [difference adjusted for gender and gestational age: -123 g (95% CI -167, -79)]. Parity, age of the pregnant women, hypertension, parental height and parental birthweight, marital status, pregnancy planning, financial concerns, number of people in household, weight gain and smoking habits individually explained part of the differences in birthweight, while adjustment for working hours and body mass index resulted in increases in birthweight differences between the educational levels. After full adjustment, the difference in birthweight between lowest and highest education was reduced by 66%. Our study confirmed remarkable educational inequalities in birthweight, a large part of which was explained by pregnancy characteristics, anthropometrics, the psychosocial and material situation, and lifestyle-related factors. Altering smoking habits may be an option to reduce educational differences in birthweight, as many lower-educated women tend to continue