58 Chapter 3 Our population showed a relatively high BPD incidence. The definition used for BPD, categorizes BPD severity according to the respiratory support at 36 weeks PMA which could lead to the inclusion of infants who had a relatively short overall duration of respiratory support. 13 Previous studies and several clinics still use the older definition criteria which define BPD as having had a need for supplemental oxygen for at least 28 days. 30 Using the older criteria 8 infants no longer would have been classified as having BPD, while 9 infants classified as not having BPD would have been classified otherwise. Using the definition by Job et al. our results still showed a similar pattern with regards to the relationship between IGF-I, BPD and nutrition. In addition, the incidence in our population would have stayed relatively high despite average antenatal corticosteroid use 31 and a short duration of mechanical ventilation. Moreover, only a limited number of extremely preterm infants were enrolled, limiting the statistical power of analyses below 30 weeks PMA. Lastly, it remains noteworthy that causality could not be examined due to the observational nature of this study. Conclusions In conclusion, this study provided further insight into the relationship between the development of the endocrine axis and the occurrence of BPD: low IGF-I levels and a slower increase in IGF-I are associated with the development of BPD. Furthermore, this study pointed towards a possible effect of donor human milk on the development of BPD. However, more research is needed to investigate whether there truly is an increased risk of BPD in infants fed donor human milk.
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