Catch up growth in preterm infants.

Seventy-one surviving infants were followed up from birth to 24 weeks of postnatal age. Their mean gestational age was 32 weeks with a range of 26-36 weeks and a standard deviation of 2.1 weeks. Their mean birth weight was 1,805 kg with a range of 0.675-2.5 kg and a standard deviation of 0.408 kg. Their weights, lengths and head circumferences were measured at birth, 6, 12 and 24 weeks. Curves for the mean weight, length and head circumference were produced and superimposed on the available intrauterine and extrauterine growth charts. The growth curves of the preterm infants did not show the flattening noted in the intrauterine curves towards term. The curve of the mean weight of the preterm infants started at the 50th centile for Gairdner & Pearson (1971) at birth to drop below that shortly after birth. At 40 weeks of postconceptional age the mean weight curve of preterm infants crossed the 50th centile and continued above it to reach the 90th centile at 60 weeks. The curves of mean length and head circumference started below the 50th centile at birth and crossed it at 40 weeks and continued above it to approach the 90th centile at 60 weeks. Growth velocity was calculated as a relative gradient using the straight line equation (y = a + bx), where y is the weight, length or head circumference, and x is the independent variable and here it is the group mean of the parameter at the corresponding ages. Catch up growth is taken as a relative gradient significantly greater than one. The first 24 weeks of postnatal life are defined as a period of catch up growth with the first 8 weeks as an interval of maximum head velocity.