This is a follow-up study to investigate the long-term safety and efficacy of human umbilical cord -derived mesenchymal stem cells (hUC-MSCs), for the treatment of BPD in premature infants. Subjects who participated in and completed the initial stage of the PhaseⅠtrial (NCT03558334 ) will be followed-up until 48 months after the hUC-MSCs transplantation.

Subjects who completed the initial stage of the PhaseⅠclinical trial will be followed-up at 5 additional visits: 1, 3, 6, 12 and 24 months after the hUC-MSCs transplantation.

Inclusion Criteria:

1. Subject who completed the safety and efficacy evaluations in hUC-MSCs PhaseⅠ clinical trial
2. Subject with a written consent form signed by a legal representative or a parent

Exclusion Criteria:

1. Subject whose parent or legal representative does not agree to participate in the study
2. subject who is considered inappropriate to participate the study by the investigator