Objective: To establish the role of individual definition of smallness at birth in the association between birth weight and long-term metabolic outcomes. Methods: Lipid profile and oral glucose tolerance test were performed in young adults (22 years) born either small (SGA) or appropriate for gestational age (AGA). AGA/SGA were defined by both population-based and customized methods adjusting for individual maternal/pregnancy characteristics. 825 individuals were classified as AGA and 575 as SGA by both methods, 131 were SGA by the population-based method only (SGA<sub>pop</sub>) and 22 were SGA by the customized method only (SGA<sub>cust</sub>). Results: SGA<sub>cust</sub> subjects had higher total cholesterol and triglyceride levels and lower high-density lipoprotein cholesterol concentrations than SGA<sub>pop</sub> and AGA subjects, however, insignificantly when adjusted for age, gender and body mass index. The homeostasis model assessment for insulin resistance (HOMA-IR) index was higher in the SGA<sub>cust</sub> (p = 0.05) and SGA<sub>pop</sub> (p = 0.02) versus the AGA group. Controlling for the HOMA-IR index, the insulinogenic index was significantly lower in the SGA<sub>cust</sub> versus SGA<sub>pop</sub> (p = 0.001) and AGA (p = 0.003) groups. In SGA<sub>cust</sub> individuals, the HOMA-IR index was clearly shifted to higher, while the insulinogenic index to lower tertiles of AGA distribution; SGA<sub>pop</sub> subjects had the HOMA-IR and insulinogenic index predominantly in the highest tertiles. Conclusions: Individualized birth weight standards allow to better identify subjects who failed to reach their genetic potential of intrauterine growth and are at higher risk of metabolic disturbances and impaired insulin secretion later in life.