Maternal concentrations of insulin-like growth factor I (IGF-I) and insulin-like growth factor binding protein 1 (IGFBP-1) may influence fetal growth. Offspring birth weight related to maternal IGF-I and IGFBP-1 measured in pregnancy was studied in 368 randomly selected women without preeclampsia who delivered a singleton liveborn child in Norway between 1992 and 1994. Maternal IGF-I concentrations were not consistently associated with birth weight, but a 1-standard deviation stronger increase in IGF-I from the first to second trimester was associated with an 82-g (95% confidence interval (CI): 11, 153) higher birth weight. IGFBP-1 concentrations were inversely associated with birth weight: Birth weight was 71 g (95% CI: 14, 128) lower per 1-standard deviation higher IGFBP-1 in the second trimester, and an increase in IGFBP-1 from the first (below median) to second (above median) trimester was associated with a 342-g (95% CI: 124, 560) lower birth weight, compared with having low IGFBP-1 (below median) in both trimesters. Conversely, low IGFBP-1 in both trimesters was associated with a 200-350-g higher birth weight compared with other combinations of IGFBP-1. In conclusion, persistently low IGFBP-1 in pregnancy is associated with relatively higher birth weight. Maternal insulin resistance may provide a link between IGFBP-1 and offspring birth weight.