Study patients will be randomized at birth to either placebo or probiotic administration (treatment group). The hospital pharmacy will be in charge of random allocation. Vials of probiotic and placebo will only be identified by the pharmacist and according to randomization will be sent to the NICU for administration to the patient. For those randomized to the treatment arm, L reuteri DSM 17938 will be administered at a dose of 10 to the eighth colony-forming units in 5 drops of a commercially available oil suspension once per day until discharge from the hospital. This oil suspension is stable for 21 months at 2o C to 8o C (as documented by the manufacturer, BioGaia AB, Stockholm, Sweden). For those randomized to the placebo arm, patients will receive an equal number of drops from an identical vial containing only the oil base, but without the probiotic. The above preparations will be administered even if the patient has not begun a feeding protocol if there are no contraindications for feeding. Patients will be stratified according to their exposure to breast milk or premature infant formula alone or a combination of both. Infants of mothers with insufficient production of breast milk will be offered premature formula.