Association of birth weight Z-score with bronchopulmonary dysplasia or mortality in very preterm infants: a cohort study

BACKGROUND: Birth weight (BW) Z-score is associated with outcomes in very preterm infants (VPIs). This study aimed to investigate the association between BW Z-score and the adverse outcomes in VPIs. METHODS: This retrospective cohort study included VPIs admitted to a tertiary neonatal intensive care unit between 1 January 2014 and 31 December 2023. Restricted cubic splines and multivariable logistic regression models were employed to assess associations between BW Z-score and primary outcomes. Infants were categorised based on the identified turning point of Z=-0.35 in the Z-score distribution, where the risk gradient changed most sharply. The primary outcome was bronchopulmonary dysplasia (BPD) or mortality at 36 weeks postmenstrual age or discharge. RESULTS: Among 4632 included VPIs, a turning point at Z=-0.35 was identified. Compared with those with Z≥-0.35, VPIs with Z