Insulin-like growth factor-binding protein-1 (IGFBP-1) is believed to be an inhibitory factor for fetal growth. The regulation of IGFBP-1 secretion in the fetus is uncertain, although insulin and oxygen tension (PO 2 ) and saturation are thought to play a role. We studied IGFBP-1 levels in umbilical cord artery (UA) and vein (UV) of 98 singleton fetuses at term with clinical signs of distress during labor, i.e. meconium-stained liquor or/and an abnormal fetal heart rate tracing. Blood gas

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... and serum C-peptide and IGFBP-1 concentrations were measured in both UA and UV. Twenty-five fetuses had an UA pH<7•20. The concentrations of IGFBP-1 were similar in UA and UV and were highly correlated (r=0•98). IGFBP-1 levels were inversely correlated with birth weight, with increased concentrations in small-for-gestational age fetuses (c10th weight percentile). IGFBP-1 levels were negatively correlated with C-peptide concentrations, and remained so after correction for birth weight (r= 0•37 for both UA and UV; P<0•001); more specifically, IGFBP-1 levels were increased in the lowest C-peptide quartile (<0•23 nmol/l) compared with the other quartiles. In addition, IGFBP-1 levels were inversely correlated with PO 2 values (r= 0•39 in UA and r= 0•34 in UV; P<0•001); quartiles of UA and UV PO 2 showed a gradual increase in IGFBP-1 concentrations with lower PO 2 values. A regression model with C-peptide and PO 2 values as independent variables predicted IGFBP-1 concentrations (R 2 of model was 0•25 and 0•22 for UA and UV respectively; P<0•001). Other blood gas values (pH, PCO 2 , HCO 3 and base deficit) did not correlate with IGFBP-1 levels. The data of this study indicate that serum IGFBP-1 levels in term fetuses are determined by both insulin and PO 2 levels, and suggest that acute hypoxemia stimulates IGFBP-1 secretion in the fetus.