The recording or bowels is easy and cheap. The investigators wonder if these sounds are modified in babies with high risk of necrotizing enterocolitis. In this study, the investigators suggest to record and do a spectral analysis of 30 seconds of bowel sounds in preterm babies of less than 32WA before and after enteral nutrition, every day until the end of hospitalization. A spectral analysis will be made for each record to determine frequencies of the signal. The investigators will try to determine physiological frequencies and look for modifications in pathological situations.