The extent to which maternal folate and vitamin B12 modulate infant birth weight is unclear. The present study investigated mothers in early gestation (mean 11·5 (sd 5·8) weeks) and neonates, at delivery. Erythrocyte (RBC) folate (mothers: n 683; neonates: n 614) and vitamin B12 (mothers: n 534; neonates: n 614) were measured. Data on smoking habits were available for 44 % of pregnancies (n 443). The relationship between vitamin levels and birth weight standardized for gender and gestational age was investigated, using linear regression and adjusting for possible confounding variables (maternal age, parity). Results are presented as standardized regression coefficients (b). Increasing maternal age was associated with elevated RBC folate (b 0·11 (95 % CI 0·08, 0·15), P<0·001; n 674) and smoking was associated with a decrease in maternal RBC folate (b −1·38 (95 % CI −1·92, −0·86), P=0·001; n 319). Neonatal RBC folate was predicted by maternal RBC folate (b 0·08 (95 % CI 0·04, 0·11), P=0·001; n 315) and maternal vitamin B12 (b 0·08 (95 % CI 0·01, 0·16), P=0·02; n 252). Smoking influenced maternal vitamin B12 status (b −0·88 (95 % CI −1·49, −0·27), P=0·005; n 231). Using univariate regression, smoking significantly influenced infant birth weight (b −2·15 (95 % CI −3·24, −1·04), P<0·001; n 437). However, the effect of smoking on birth weight was statistically non-significant when considered in a multivariate regression model, leaving maternal RBC folate as the only significant predictor of birth weight (b 0·25 (95 % CI 0·08, 0·42), P=0·005; n 145). These findings suggest that maternal folate status is an important determinant of infant birth weight. The combined effects of smoking and reduced RBC status on birth weight require further investigation.