Objective We compared mortality and morbidity of inborn versus outborn very preterm infants <32 weeks’ gestation in Western Australia (WA) between 2005 and 2018.

Design Retrospective cohort study.

Patients Infants <32 weeks’ gestation who were born in WA.

Main outcome measures Mortality was assessed as death before discharge home from the tertiary neonatal intensive care unit. Short-term morbidities included combined brain injury (intracranial haemorrhage grade ≥3 and cystic periventricular leukomalacia) and other major neonatal outcomes. Developmental assessments at age 2, 3 and 5 years were evaluated. We performed multivariable logistic regression analysis of outborn status on outcomes, controlling for gestational age, birth weight z-score, sex and multiple birth.

Results A total of 4974 infants were born in WA between 22 and 32 weeks’ gestation between 2005 and 2018 of which 4237 (89.6%) were inborn and 443 (10.4%) were outborn. Overall mortality to discharge was higher in outborn infants (20.5% (91/443) vs 7.4% (314/4237); adjusted OR (aOR) 2.44, 95% CI 1.60 to 3.70, p<0.001). Outborn infants had higher rates of combined brain injury than those inborn (10.7% (41/384) vs 6.0% (246/4115); aOR 1.98, 95% CI 1.37 to 2.86), p<0.001). No difference in up to 5-year developmental measures was detected. Follow-up data were available for 65% of outborn and 79% of inborn infants.

Conclusions Outborn preterm infants <32 weeks in WA had increased odds of mortality and combined brain injury than those inborn. Developmental outcomes up to 5 years were similar between groups. Loss to follow-up may have impacted the long-term comparison.