About 5% of newborns are small for gestational age (SGA) and 10-15% of them do not naturally catch up on growth by 2 yr of age. The growth of the fetus from conception to birth results from complex interactions of maternal and fetal genes with the environment, and factors such as malnutrition are well known to influence fetal growth. Specific genetic disorders such as Leprechaunism, Bloom syndrome, Fanconi anemia are inherited, but are very rare causes of intrauterine growth retardation. Recent published research on the actions of IGF-I in humans and the phenotypes of children with genetic defects in the GH/IGF axis establish IGF-I signaling via its receptor (IGF-IR) as the critical growth-controlling element in man. The aim of this article is to review certain SGA disorders of Mendelian genetic origin, with an emphasis on defects in the insulin and IGF pathways which may be implicated in the persistence of short stature in some children born SGA.