In this context, even if the CO-Hb levels may be underestimated in the moderate-to-severe BPD group compared to the no-or-mild BPD group, our findings suggested that CO-Hb levels measured during the early postnatal period can be substituted for ETCO measurements and that they may predict the development of BPD. Here, GSTM1 is linked to bronchopulmonary dysplasia.