The energy requirements for growth can well be studied in the preterm neonate. The rate of weight gain during the last trimester of normal pregnancy is higher than during any other period of human life, 15–17 g/kg/d. The weight of a normal fetus triples in the 10-week period from 30 to 40 wk of normal pregnancy.

At birth a preterm infant is classified, according to the combination of birthweight and gestational age, as appropriate, small or large for gestational age. Several studies have shown metabolic differences between these groups. Measurements of both weight and gestational age however should be considered with caution. An increase in total body water causes an increase in weight. Recently we observed an increase in bodyweight of infants whose mothers received indomethacin to stop labour, the higher weight being probably only caused by an increase in water. Growth therefore is not equal to a gain in bodyweight, and classification as small, appropriate or large for gestational age should really be based on measurements of bodycomposition and not only on measurements of weight, length and gestational age.

Changes in bodycomposition during the last trimester of normal pregnancy are impressive, as shown in Figure 1. The increase in protein is almost linear, while fat is deposited mainly toward the end of pregnancy. The percentage of total body water declines during the last trimester.

It is questionable if the changes in body composition of the fetus are representative of the growing preterm infant. Data on the body composition of the fetus have been collected from infants who died around birth. Secondly, the curves are constructed from cross-sectional data and not from longitudinal studies, which might better represent growth.