The purpose of this study is to know the effects of probiotics on the incidence of Necrotizing Enterocolitis (NEC) in preterm infants less than 1500 g.

The investigators performed a clinical, randomized, double-blind trial in preterm infants <1500g from March 1, 2010 to February 28, 2012. Group A received enteral stimulation trophic five days and later increments 20ml/k/day of breast milk or formula for premature; the group B apart from the above the probiotic Lactobacillus acidophilus (boucardii strain) 1 X109 colonic forming unit (CFU)/ day for 4 weeks was added. Gastric tolerance, evolution, reason for discharge, and the development of Necrotizing enterocolitis was monitored. The diagnosis of Necrotizing enterocolitis was made by the treating physician according to Bell's criteria and was blinded to group assignments.The formulas were prepared by one of the investigators according to the group each patient was assigned. Using sterile technique, the probiotic with 1x109 (CFU) Lactobacillus acidophilus (boucardii strain) was added at a dose of 125 mg/ kg/dose twice daily for 4 weeks to fresh supplement breast milk or formula for preterm infants. In case of control group only breast milk or formula for preterm infants was used.

All formulas were labeled for each patient and were delivered to the nurse in charge of infant feeding at 9:00 and 21:00 h.

Characteristics of Sample size and statistical analysis:

Sample size showed that the investigators needed 190 total preterm infants, based on the expected 50% decrease in the incidence of severe necrotizing enterocolitis in preterm neonates who received probiotics compared with control group with α 0.05 and β 0.20. Student´s t test or Mann Whitney U test were performed for mean differences regarding the general characteristics in the two study groups, Chi square or Fisher´s test for differences in proportions for mortality and complications between groups.

Inclusion Criteria:

• We studied 150 preterm newborns <1500gr who were recruited on the day that began enteral feeding according to the decision of the treating physician.

Exclusion Criteria:

• Preterm infants > 1500g with Apgar score <6 at 5 minutes, gastrointestinal malformations, fetopathies, patent ductus arteriosis with hemodynamic decompensation, asphyxia, and NEC suspected as classified by Bell were excluded from the study.