In the beginning of the 20th century, the only explanation that was offered to a small baby born was that of prematurity. Fetal growth was defined by birth weight alone. The concept that a fetus might suffer poor growth in utero became recognized in the 1960's. In 1963 Lubchenco and colleagues showed that the classification of neonates by birth weight percentile had a significant prognostic advantage. This classification improved the detection of neonates with intra uterine growth restriction and those at risk of adverse health events throughout life.