To evaluate, in a prospective multicenter study, ultrashort-term heat inactivation for the prevention of Cytomegalovirus (CMV) transmission in preterm infants (<32 weeks gestational age or <1500 g birth weight) under clinical conditions. Inactivation will be done only during the period of infectivity of breast milk, characterized by viral excretion strongly associated with subsequent infection, monitored by periodic virologic examinations of BM and urine of the infant. Thus the investigators hypothesis is that no CMV transmission through breast milk will occur using a gentle ultrashort heat inactivation procedure applied to infective breast milk.