Objective To compare the risk of mortality and morbidity between outborn and propensity score-matched inborn extremely preterm neonates.

Setting Multiple neonatal intensive care units (NICU) across the USA.

Patients Singleton neonates born at 22–29 weeks’ gestation with no major anomalies who were admitted to a NICU and discharged between 2000 and 2014. Outborn neonates were restricted to those who transferred into a NICU on the day of birth.

Methods The association between inborn-outborn status and the time-to-event outcomes of in-hospital mortality and necrotising enterocolitis (NEC) were assessed using Cox proportional hazards regression. Logistic regression was used to assess the remaining secondary outcomes: retinopathy of prematurity requiring treatment (tROP), chronic lung disease (CLD), periventricular leucomalacia (PVL) and severe intraventricular haemorrhage (IVH). Since outborn status was not random, we used 1:1 propensity score matching to reduce the imbalance in illness severity.

Results There were 59 942 neonates (7991 outborn) included in the study. Outborn neonates had poorer survival than inborns and higher rates of NEC, severe IVH, tROP and PVL. Inborn-outborn disparities in mortality were reduced over the study period. When analysing the matched cohort (6524 matched pairs), outborns were less likely to die in-hospital compared with inborns (HR 0.84, 95% CI 0.77 to 0.91). However, outborns experienced higher rates of NEC (HR 1.14, 95% CI 1.04 to 1.25), severe IVH (OR 1.52, 95% CI 1.38 to 1.68), tROP (OR 1.45, 95% CI 1.25 to 1.69) and CLD (OR 1.12, 95% CI 1.01 to 1.24).

Conclusion Additional research is needed to understand the contributors to increased morbidity for outborn extremely preterm neonates and identify interventions that mitigate this risk.