Adiponectin is an adipocyte-derived hormone with profound insulin-sensitizing, antiinflammatory, and antiatherogenic effects. Apart from its obvious potential as a mediator of adult metabolic syndrome, adiponectin could have a significant role in regulating fetal growth.We measured plasma adiponectin concentrations by ELISA in cord vein of 197 infants. Of them, 122 were born preterm (gestational age, 22-32 wk), and 75 at term (49 from a healthy and 26 from a diabetic pregnancy, with similar findings, and thus all data from term infants pooled).Mean adiponectin concentrations increased from less than 1 microg/ml at 24 wk gestation to approximately 20 microg/ml at term. One week increase in gestational age corresponded in preterm infants to 43% increase (95% confidence interval 34-53%; P < 0.0001) in adiponectin and term infants to 21% increase (12-31%; P < 0.0001). In preterm infants, one unit increase in birth weight sd score corresponded to 42% increase (22-66%; P = 0.0001) in adiponectin, and females had 57% higher adiponectin concentrations (0-146%; P = 0.05) than males. These differences were not seen in term infants. Adiponectin levels were lower in preterm infants with recent (<12 h) exposure to maternal betamethasone but were unrelated to mode of delivery, preeclampsia, or impaired umbilical artery flow. In conclusion, adiponectin concentrations in fetal circulation show a 20-fold rise between 24 wk gestation and term and, in preterm infants are associated with birth weight sd score, sex, and glucocorticoid exposure. Adiponectin may play an important role in regulating fetal growth and explaining its links to the metabolic syndrome and its consequences during adult life.