Previous data suggested that small for gestational age newborns have increased levels of IGF binding protein 1 (IGFBP1) in amniotic fluid (AF) at 15-16 wk of pregnancy. In this study, we developed an RIA for IGFBP1 and measured IGFBP1 concentrations in 209 AF samples with normal fetal karyotype between 14 and 20 wk; we measured IGF-I, IGF-II, and C-peptide in the same samples. Concentrations of these growth-modulating factors were all positively correlated with gestational age at sampling (p< 0.0001). After correcting for gestational age, AF IGFBP1 remained strongly correlated with IGF-I and IGF-II (both p< 0.0001); their concentrations were many times higher in AF than in cord serum during the third trimester. None of the growth-modulating factors in AF correlated with birth weight, after correction for gestational age; birth weight percentile distribution was comparable in two groups of newborns who had AF values of IGF-I, IGF-II, IGFBP1, or C-peptide that were either less than or equal to the 50th percentile or more than the 50th percentile at sampling. However, placenta weight and the placenta weight to birth weight percentage were negatively correlated with AF IGF-I, IGF-II, and IGFBP1; placenta weight to birth weight percentage was lower in pregnancies with IGFBP1 values more than the 50th percentile compared with those less than or equal to the 50th percentile at sampling. In conclusion, AF concentrations of IGFBP1 increase gradually between 14 and 20 wk gestational age and correlate with IGF-I and IGF-II levels; high IGFBP1 levels do not predict small for gestational age newborns, but are associated with lower placenta weight.