Significance of low birthweight for gestational age among very preterm infants.

Objective: To estimate the risk of specific adverse neonatal events resulting from the combined effects of prematurity and low birthweight in very preterm infants (delivered at 24-31 weeks of gestation).

Design: A cohort study of specific adverse neonatal events in preterm infants born at between 24 and 31 weeks of gestation.

Setting: Pavia, Italy.

Population: Two hundred and thirty singleton infants with sonographically confirmed gestational age, delivered at 24 to 31 weeks of gestation.

Methods: To evaluate the impact of a lower than expected birthweight on selected neonatal events independently of gestational age, we calculated birthweight standard deviation scores (differences between actual birthweight and fitted birthweight divided by fitted standard deviation) for each week of gestation.

Results: After adjustment for gestational age and other confounders, there was a significant linear trend relating a decreasing birthweight SDS to an increased likelihood of neonatal death, intraventricular haemorrhage, severe respiratory distress syndrome, and acidosis. Compared with infants with SDS > or = 0 (> or = 50th centile of birthweight), infants with birthweight SDS < -1 (< 16th centile) had increased odds for neonatal death [odds ratio (OR) 3.7, 95% confidence interval (CI) 1.42-9.6], grade III-IV intraventricular haemorrhage (OR 17.5, 95% CI 4.04-75.9), and neonatal acidosis (OR 3.22, 95% CI 1.41-7.4). The significance of birthweight SDS as a predictor of neonatal outcome, however, was lower than that of gestational age.

Conclusions: A lower than expected birthweight affects the likelihood of several adverse neonatal events in very preterm infants. However, a decreasing birthweight SDS affects neonatal outcome less than decreasing gestation does.