Inclusion Criteria:  1. born at study hospital; 2. monozygotic twins less than 32 weeks GA. 3.Signed informed consent obtained; 4. available and qualified umbilical cord blood (UCB).  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.