Birth-weight centiles and the risk of serious adverse neonatal outcomes at term

Birth-weight is an important determinant of perinatal outcome with low birth-weight being a particular risk factor for adverse consequences.

To investigate the impact of neonatal sex, mode of birth and gestational age at birth according to birth-weight centile on serious adverse neonatal outcomes in singleton term pregnancies.

This was a retrospective cohort study of singleton term births at the Mater Mother's Hospital, Brisbane, Australia. Serious adverse neonatal outcome was defined as a composite of severe acidosis at birth (pH ≤7.0 and/or lactate ≥6 mmol/L and/or base excess ≤-12 mmol/L), Apgar <3 at 5 min, neonatal intensive-care unit admission and antepartum or neonatal death. The main exposure variable was birth-weight centile.

Of the 69,210 babies in our study, the overall proportion of serious adverse neonatal outcomes was 9.1% (6327/69,210). Overall, neonates in the <3rd birth-weight centile category had the highest adjusted odds ratio (OR) for serious adverse neonatal outcomes [OR 3.53, 95% confidence interval (CI) 3.06-4.07], whilst those in the ≥97th centile group also had elevated odds (OR 1.51, 95% CI 1.30-1.75). Regardless of birth modality, smaller babies in the <3rd centile group had the highest adjusted OR and predicted probability for serious adverse neonatal outcomes. When stratified by sex, male babies consistently demonstrated a higher predicted probability of serious adverse neonatal outcomes across all birth-weight centiles. The adjusted odds, when stratified by gestational age at birth, were the highest from 37+0 to 38+6 weeks in the <3rd centile group (OR 5.97, 95% CI 4.60-7.75).

Low and high birth-weights are risk factors for serious adverse neonatal outcomes. The adjusted OR appears to be greatest for babies in the <3rd birth-weight centile group, although an elevated risk was also found in babies within the ≥97th centile category.