Because at some point during the first week of life almost every newborn has a total serum bilirubin (TSB) level that exceeds 1 mg/dL (17 μmol/L, the upper limit of normal for an adult), and two-thirds or more of newborns will appear clinically jaundiced, this type of transient hyperbilirubinemia has been called physiologic jaundice. This jaundice results from the interaction of a number of factors (see Table 35-3). The term physiologic jaundice generally is applied to newborns whose TSB level falls within the normal range, but because of the significant differences in TSB levels in different populations, it can be difficult to define what is normal or abnormal, physiologic or nonphysiologic. Additionally, defining the term “normal” is, in itself, a difficult task, and depends on whether one chooses a gaussian, percentile, diagnostic, risk factor, or therapeutic definition of the term (35).