Amniotic fluid C-peptide (AFCP) was monitored as an indicator of the amount of insulin secreted by the fetus in utero. Levels of amniotic fluid and cord blood C-peptide, insulin, and glucose were measured in 103 nondiabetic infants at greater than or equal to 36 weeks's gestation. Infants were grouped according to birth weight and gestational age at delivery, as follows: small gestational age (SGA, less than or equal to 10%, n = 11), average for gestational age (AGA, 10% to 90%, n = 75), large for gestational age (LGA, greater than 90%, n = 17). AFCP correlated best with infant weight-gestational age percentile classification: low AFCP in SGA infants and high AFCP in LGA infants. The data from this study suggest that a persistently low production of insulin by SGA fetuses and a high production of insulin by LGA fetuses may lead to the different intrauterine growth rates observed.