Inclusion Criteria: (1) born at the study hospital; (2) singleton birth; (3) 26<GA <32 weeks; (4) free of severe congenital anomalies or genetic syndromes;  Exclusion criteria： (1) mothers with clinical chorioamnionitis; (2) the mother was opsitive for hepatitis B (HBsAg and/or HBeAg), hepatitis C (anti-HCV), syphilis, HIV (anti-HIV-1 and -2), and IgM against cytomegalovirus, rubella, toxoplasma, and herpes simplex viruses; (3) consent was not obtained from the parents or guardians; and (4) after processing, UCB cells were not available.