Fetal macrosomia is an heterogeneous condition in terms of definition and etiologic factors. Recent findings suggest that macrosomia should not be classified on the basis of birth weight and gestational age alone. The ponderal index delineates a symmetric and an asymmetric subtype of macrosomia. The relationship between maternal diabetes and fetal macrosomia has been extensively investigated. However, eighty percent of macrosomic infants are born to mothers who are not hyperglycemic, and various factors have been associated. Maternal factors explain approximately 50% of the variance in birth weight, whereas paternal factors have no significant effect. The predisposition to excessive fetal growth may be shared within the intra uterine environment and the fetal genome. The respective roles of lipids, amino acids, hormones such as leptin, and growth factors need to be evaluated. Perinatal morbidity and long term consequences such as obesity and glucose intolerance might be associated with some of the factors leading to fetal overgrowth.