Infants who have low platelets and who require a platelet transfusion are included in this study. Platelet transfusions are routinely given to infants when their platelet count falls below a certain level. The study will look at the amount of platelets transfused. The purpose of the study is to evaluate the effect of platelet transfusions on the level of a protein (thrombopoietin) which is known to help control platelet production.