Length and ponderal index at birth: associations with mortality, hospitalizations, development and post-natal growth in Brazilian infants

Abstract

Background: Low birthweight infants suffer greater mortality and neonatal morbidity, grow less well in infancy and show poorer psycho-motor development. However, this simple categorization may obscure important differences in aetiology and prognosis between infants born stunted, thin, or both.

Methods: In 1993, all births in Pelotas, Brazil, were enrolled into a prospective study of health and development in infancy. Of 5249 live births, 5160 had length and weight measures at birth, and were classified into tertiles of length and ponderal index. All deaths and hospitalizations were monitored, and suspected developmental delay and attained growth at 12 months were assessed on a subsample of 1364 infants. Logistic regression was used to control for gestational age and socioeconomic status.

Results: There was no association between birth length and ponderal index tertiles. After adjusting for gestational age, infants in the lower tertiles of both length and ponderal index presented a 3.8-times higher risk of mortality from day 8 to day 365, and a 2.5-times higher risk of hospitalization compared to infants with greater birth lengths and/or ponderal indices. Suspected developmental delay was associated with length and, less strongly, with ponderal index, but there was no synergism between the two. Infants in the middle and upper tertiles of ponderal index at birth became thinner.

Conclusions: Birth length was strongly associated with development at 12 months, but only infants born both short and thin were at increased risk of mortality and hospitalizations. The combination of the two measures provides a useful classification of the anthropometric status of the newborn.

PIP: A prospective study of all 5249 live births in Pelotas, Brazil, in 1993 examined interactions between health and development in infancy. The 5160 infants who had length and weight measurements taken at birth were classified into tertiles of length-for-age Z score and ponderal index--a measure of soft tissue growth. There was no association between these two measures. After adjustments for gestational age, infants in the lower tertiles of both length and ponderal index had a 3.8 times greater risk of mortality from day 8 to 365 and a 2.5 times higher risk of hospitalization than infants in the high tertile. However, infants born short but not thin, or thin but not short, were not at increased risk of either mortality or hospitalization. Suspected developmental delay at 12 months was associated with shorter stature at birth and, less strongly, with a lower ponderal index, but there was no synergism between the two measures. Infants in the middle and upper tertiles of ponderal index at birth became markedly thinner in the first year of life, while length changes were less strongly associated with initial status. Overall, these findings suggest that the combination of length and ponderal index at birth may provide a functionally relevant means of classifying the newborn's anthropometric status since the two measures are relatively independent of each other at the level of the individual and appear to affect different aspects of the infant's subsequent health and development.