A large RCT using a single probiotic, Bifidobacterium breve, in 23-30 week gestation babies, showed no difference in the outcomes of NEC, sepsis or death before discharge.
Reference: Costeloe, K., Hardy, P., Juszczak, E., Wilks, M. and Millar, M.R., 2016. Bifidobacterium breve BBG-001 in very preterm infants: a randomised controlled phase 3 trial. The Lancet; 387 (10019): 649-660. (1315 infants).