Two methods for estimating age of newborns in catch-up growth studies

Catch-up growth has been the subject of many studies, but whether and when infants born preterm and with low birth weight achieve normal size is controversial. One of the reasons for diverse results is likely to be the difference in how the infants' ages are calculated. The aim of this work is to analyse the occurrence and chronological distribution of compensational events of 163 infants born preterm conducted according to both chronological and corrected age. Catch-up growth was defined as the compensation of body weight values to those included between +/-1 SD in the reference system. Data were analysed using the survival analysis. Catch-up growth events were observed in greater amount for gestation-adjusted age. The correction of age allows the indication of the age, corresponding to the correct time of birth, as the time of the most rapid growth velocity. Chronological age does not allow any particular time to be distinguished as the most characteristic moment for achievement of constitutional growth. Within the whole studied period there is a similar probability of growth equalisation. A description of the morphological development of preterm infants should not be made in comparison to the postnatal reference system without correction of age, but analyses considering chronological age are necessary when the influence of gestational age is considered.