Objective: To determine the independent contributions to infant birth size of insulin-like growth factor I (IGF-I) and leptin measured in umbilical cord plasma.

Methods: Umbilical cord blood was collected in 12 804 consecutive deliveries, and cord plasma from 585 singleton infants born at term after uncomplicated pregnancies was analyzed for leptin, IGF-I, and 2 IGF-binding proteins (IGFBP-1 and IGFBP-3). In multivariable analyses, we assessed maternal and infant covariates of leptin and IGF-I, and we evaluated the independent contribution of cord levels of leptin and IGF-I on infant birth size.

Results: Cord plasma levels of IGF-I were lower in women who reported smoking at the beginning of pregnancy compared with nonsmokers. In female infants, levels of IGF-I and leptin were higher than in male infants after adjustment for ponderal index and maternal factors. We found a strong parallel increase in umbilical IGF-I and leptin with increasing birth weight and birth length. For IGFBP-1, there was an opposite pattern: IGFBP-1 increased with decreasing birth size. The multivariable analysis, adjusted for length of gestation and maternal age, parity, prepregnancy weight, smoking during pregnancy, and offspring sex, showed that IGF-I and leptin, independent of each other, were associated with birth weight and birth length.

Conclusions: Levels of IGF-I and leptin in umbilical cord plasma were higher in girls than in boys, but in both sexes, these 2 factors contributed independently and positively to birth weight and length. For IGFBP-1, high levels were associated with low birth weight and reduced length. If intrauterine growth is related to the risk of developing adult diseases, IGF-I, IGFBP-1, and leptin may be involved in the underlying processes.1131-1135 insulin like growth factors, leptin, umbilical cord plasma, birth weight.