Birthweights of 3,451 infants of women registered for antenatal care in Uppsala County, Sweden, were analyzed using three different maternal socio-economic indicators; education, socio-economic status and work environment exposure characteristics. Other explanatory variables were maternal age, parity, height, smoking habits, and length of gestation. Mean birthweights increase with longer education and higher socio-economic status. No general pattern was seen for work environment characteristics. When smoking habits are controlled for, social differences in birthweight decrease to non-significant values. A regression model with a socio-economic indicator alone explains only a minor part, less than 1%, of the variation in birthweight. When smoking is included, adding a socio-economic indicator does not significantly improve the model. Practically all social differences in birthweight are related to the differences in maternal age, parity, height, and smoking habits. If a socio-economic indicator is to be included in the analysis of birthweights (for other reasons like international comparisons), we recommend education.