- To evaluate if changes of intestinal perfusion and oximetry induced by feeding are related to baseline (< 72 hours of life) values of Doppler flow velocimetry and of Near Infrared Spectroscopy (NIRS). After birth, in the first 24 hours of life, and in the transitional period, between the 48th and 72nd hours of life, all infants' intestinal perfusion will be evaluated with NIRS and a doppler of the superior mesenteric artery will be executed.