Objective: Fetuses with intrauterine growth restriction (IUGR) have adaptive hormonal changes including changes in insulin, which may increase their future risks for developing diabetes mellitus. This study compared cord blood insulin concentrations in IUGR and appropriate for gestational age (AGA) fetuses in a monochorionic (MC) twin model. Materials and methods: Ten pairs were classified as selective IUGR (sIUGR) based on having one twin weight below the 10th percentile and with an intertwin birth weight discordance>20%. Fourteen pairs without IUGR were included as a comparison group. Pregnancies with twin–twin transfusion syndrome, congenital structural malformations, and genetic abnormalities were excluded. Insulin and glucose concentrations were measured in cord venous blood at the time of delivery. Results: Cord blood insulin concentrations of sIUGR fetuses were significantly lower than those of AGA counterpart fetuses in MC twins affected by sIUGR (5.1±4.1 mU/L, range: 0.7–9.9 mU/L for sIUGR fetuses and 12.2±7.6 mU/L, range: 3.5–23.7 mU/L for AGA fetuses, p=0.019). No significant difference in insulin concentrations between larger and smaller fetuses in MC twins without IUGR was observed. Insulin concentration was inversely correlated with gestational age of delivery in all fetuses except in those with sIUGR. We did not find any difference in cord blood glucose concentrations between the two fetuses in both groups. Conclusion: Our data show reduced insulin secretion and loss of the physiological decline in concentration over time as gestational age increases in fetuses with sIUGR compared to AGA counterparts.