Maternal nutritional status, e.g. body weight and composition, is associated with fetal growth. It has been suggested that the insulin-like growth factor (IGF) system may be a mediator of this relationship. In twenty-three healthy Swedish women, we studied (1) the relationships before and during pregnancy between maternal serum concentrations of IGF-I and IGF binding protein-1 (IGFBP-1) and maternal body weight and composition; (2) interactions between serum concentrations of IGF-I (before and in early pregnancy) and maternal nutritional status in relation to infant birth weight. We found that serum IGF-I during pregnancy was positively correlated with maternal body weight (r 0·47–0·56) and fat-free body weight (r 0·61–0·65), whereas serum IGFBP-1 was negatively correlated with maternal body weight (r − 0·44 to − 0·69) and body fat (r − 0·64 to − 0·76) before and during pregnancy. Women with a lower body fat content (%) before pregnancy had greater increases in serum IGFBP-1 during pregnancy than women with a higher prepregnant body fat content (%). In addition, significant fractions of the variation in corrected infant birth weight were explained by variables related to the maternal nutritional status when these were combined with serum concentrations of IGF-I in gestational week 14 (adjusted r2 0·25–0·44, P = 0·001–0·021), but not when they were combined with such concentrations before pregnancy (adjusted r2 0·11–0·12, P = 0·105–0·121). These results suggest mechanisms by which the IGF system may be a mediator between maternal nutritional status and fetal growth.