All enrolled infants underwent LUS examination performed by two ultrasound physicians. Monitoring included a complete lung scan in the first 24 hours of life (T0) and at day 3 (T1). All neonates underwent weekly ultrasound examinations until 36 weeks postmenstrual age (T2, T3…), so that the T2 scan was carried out at the end of the first week of life. LUS was performed according to a standardized protocol [13] when the neonate was in a quiet state, during routine clinical care to minimise discomfort. All ultrasound images and videos were digitally recorded, anonymised, and reviewed by a senior independent ultrasonographer blinded to patients' clinical information. Each lung was divided into three regions according to the anterior and posterior axillary lines and the mammary line. The regions were the upper-anterior, lower-anterior, and latera regions.

The outcome was bronchopulmonary dysplasia, as defined by the NIH in 2001 and Jensen in 2019：（1）no BPD as not receiving supplemental oxygen (O2) for 28 days or at 36 weeks;mild BPD as receiving O2 for greater than or equal to 28 days but not at 36 weeks; moderate BPD as receiving O2 for greater than or equal to28 days plus treatment with less than 30% O2 at 36 weeks; and severe BPD as receiving O2 for greater than or equal to 28 days plus greater than or equal to 30% O2 or positive pressure at 36 weeks；（2）No BPD was defined as breathing in room air at 36 weeks' PMA; grade 1 BPD as receipt of nasal cannula ≤2L/min (or hood O2); grade 2 BPD as nasal cannula >2L/min, nasal continuous positive airway pressure (CPAP), or nasal intermittent positive pressure ventilation;and grade 3 BPD as invasive mechanical ventilation