Newborn size is the result of intrauterine growth. Premature, low birthweight of 2,500 g, small for gestational age (SGA, 10th percentile), or intrauterine growthrestricted (IUGR) newborns may have similar weights. Serial fetal biometry (ultrasound), required for the diagnosis, timing and severity of intrauterine growth restriction in the individual infant, is still not common in epidemiological studies. SGA newborns have less lean body mass, but they particularly lack fat mass. The most important etiological determinants of intrauterine growth restriction in developed countries is cigarette smoking, while in developing countries it is usually longstanding food deprivation. Follow-up studies of SGA newborns consistently showed a positive association between birthweight and later lean body mass, whereas associations with adiposity were more variable. Most SGA infants had catch-up in length/height. Signs of the metabolic syndrome accompanied the catch-up in bodyweight and central adiposity. So far, no overarching model is available to explain how the epigenetic and hormonal tunings, which accompany intrauterine malnutrition from preconception through pregnancy, can program the regulatory systems of fundamental life processes. The theoretical concepts of a thrifty phenotype (Hales and Barker) and of a predictive adaptive response (Gluckman and Hanson) offer a comprehensive approach to understanding the empirical and experimental findings.