This study is investigating using platelet size rather than platelet number as a guideline for transfusing platelets in infants who are hospitalized in a NICU (neonatal intensive care unit). After obtaining parental informed consent, thrombocytopenic infants will be randomized to one of two groups. 1: Transfusion based on platelet number; 2: transfusion based on a combination of platelet number and platelet mass. In each group the decision to transfuse platelets will be made using a slightly different, yet strict set of transfusion rules.