Inclusion Criteria:  1.born at study hospital; 2. singleton birth; 3. less than 28 weeks GA 4.Signed informed consent obtained; 5. Umbilical cord blood collection and testing qualified; 6.Diagnosed with BPD  Exclusion Criteria:  1. with severe congenital abnormalities; 2.with maternal clinical chorioamnionitis 3. the mother was positive for hepatitis B (HBsAg and/or HBeAg) or C virus (anti-HCV), syphilis, HIV (anti-HIV-1 and -2) or IgM against cytomegalovirus, rubella, toxoplasma and herpes simplex virus.