Our study showed an increase in all maternal interventions over the last decade. There was an increase in antenatal risk factors, including maternal age, diabetes, chorioamnionitis, premature prolonged rupture of membranes, antibiotic treatment, and cesarean sections. Protective treatment in the form of antenatal steroids and MgSO4 increased, as did the practice of delayed cord clamping. While there was an increasing number of smaller and younger infants resuscitated and admitted to the NICU, this study showed that significantly fewer infants were intubated at resuscitation. The use of mechanical ventilation decreased; noninvasive ventilation increased, and mortality decreased from 17% to 6%.

Reassuringly, most in-hospital outcomes remained stable or improved. Fewer babies underwent surgery for PDA, and stable numbers of surgeries were required for NEC and ROP. The length of hospital stay also remained stable. The incidence of major IVH and PVL in survivors remained the same, as did infants who required respiratory support at home. Taken together, these outcomes showed stable rates of survival without major morbidity. All these outcomes remained stable despite a significant increase in the number of babies born before 26 weeks of gestational age and a steady rise in antenatal risk factors.

Evaluating factors associated with CLD, we found that any parameter related to mechanical ventilation increased the patients’ odds of developing CLD. Of note, there was a significant association between any time spent on HFNP and the likelihood of developing CLD. Administration of surfactant was unexpectedly associated with increased odds of developing CLD. Also demonstrated was the lack of association between any of the antenatal risk factors and treatments, such as chorioamnionitis, antenatal steroids, and developing CLD.

The changes in maternal health were striking, with an increase in almost all antenatal risk factors for adverse neonatal outcomes. Some of these results can be explained by looking at the shift in maternal demographics such as age and weight [10]. Other considerations may relate to increasing obstetric surveillance and more obstetric interventions to prevent preterm birth, such as cervical cerclages and better and more rigorous ways to diagnose conditions such as gestational diabetes and chorioamnionitis [11]. Furthermore, over time, more infants in the lower gestational age bracket were resuscitated and survived, especially infants who were not offered resuscitation and did not survive and infants whose mothers often presented with more risk factors at birth.

Finding an increase in CLD in our population was disappointing. However, measuring CLD as a dichotomous outcome of oxygen requirement at a certain age has been debated for some time now [12, 13]. Other studies have reported a stagnant or even increasing incidence of CLD over time [14, 15]. Interestingly, in our study, the rate of infants discharged on oxygen or CPAP remained stable despite a population where more lower gestational age infants have survived, infants who are more susceptible to adverse respiratory outcomes.

The association of surfactant administration and CLD was unexpected. However, multiple studies failed to show a decrease in CLD with routine surfactant administration [16, 17]. Surfactant could have played a role in keeping sicker and smaller patients alive who then developed CLD due to their burden of risk factors (i.e., lower gestational age, etc.).

The correlation between HFNP and CLD has been described previously. Taha et al. in 2016 reported increased odds for CLD, death, prolonged ventilation days, and length of stay in a retrospective audit of over 2000 extremely low birthweight infants receiving HFNP [18]. However, one could also argue that high flow was used as part of noninvasive ventilation in infants that in the past would have been mechanically ventilated.

Other neonatal networks have reported on their longitudinal outcomes in various epochs. Stoll et al. reported on the outcomes in Neonatal Research Network Centers in the USA and, similar to our analysis, found improved survival, an increase in CLD and obstetric interventions, and a decrease in mechanical ventilation [14]. More recently, Boel et al. presented outcomes of extremely preterm infants in the UK and found a decrease in the utilization of mechanical ventilation and an increase in the use of HFNP. They reported stable rates of IVH, CLD, ROP, and NEC [19]. A major limitation of their study and especially their results of CLD is over 60% missing data for CLD diagnosis. Additionally, in 2020, the Neonatal Network from South America (NEOCOSUR) reported more obstetric intervention, increased antenatal corticosteroids, and an avoidance of mechanical ventilation in favor of noninvasive ventilation techniques. Their rates of IVH and PVL remained stable, similar to our results. They observed a very slight decrease in the incidence of CLD in their population [20]. Comparisons to this study’s results of CLD are difficult, as their study population included all infants up to 35 weeks of gestational age.

There are several strengths to our study. Our population was well defined. It is a geographical cohort and likely to accurately reflect the Australian extremely preterm population. All data used were prospectively added to a purpose-built database and were audited by trained audit officers prior to analysis, minimizing information bias. Our results are detailed, longitudinal, without any missing data, and adjusted for confounders, and we report a confidence interval for all outcomes and show robust P values.

Our study has several limitations. Epidemiological studies cannot determine causation, and our analysis can only describe associations of changes in practice and outcomes. Furthermore, due to the design of the study, our population does not include all extreme preterm patients being cared for in the last decade. The number of babies resuscitated at < 24 weeks of gestation and < 500 g birthweight is steadily increasing but was very low in the early 2010s [21]. Furthermore, the treatment and management of these babies are still widely variable in the included eight NICUs. Including these babies would have led to significant outliers and affected the validity of our results. Our study also cannot control changes in diagnostic criteria or management policies; we can only describe the change in practice that is measured. One of these changes could have influenced the incidence of some outcome measures, including our primary outcome—chronic lung disease. Changes in oxygen targets were slowly implemented during the study period as a result of randomized controlled trials such as BOOSTII, leading to higher targets and consequently potentially more oxygen use [22]. One further limitation is the inclusion of 2020. The global pandemic has had an impact on the incidence of preterm birth worldwide [23]. Our study cannot account for the impact the global pandemic had on the management and outcomes of our patients.

In summary, we identified that a clinical practice shift toward less invasive ventilation and avoidance of mechanical ventilation was temporally associated with an unchanged incidence of home respiratory support despite increasing antenatal risk factors, decreasing gestational age, and increasing survival. When providing less invasive ventilation, the use of HFNP should be considered with caution, as our results showed a potential to increase the odds of developing CLD in this population. No other change in management had a significant impact on our primary outcome. This study did not examine the impact of these measures on mortality, an outcome that significantly decreased over the last decade. Reassuringly, the incidence of major secondary outcomes such as ROP, NEC, PDA surgery, and length of stay either remained stable or decreased despite a steady decrease in gestational age. The results of this study add to the ever-growing body of evidence indicating that a shift away from mechanical ventilation might improve respiratory morbidity. Since CLD was by far the most important contributor to mortality after 36 weeks PMA, clinicians should consider these results in their daily practice.