When hyperbilirubinemia increases past a mild level, it leads to jaundice, raising the risk of progressing to kernicterus. When this happens in adults, it is usually because of liver problems. Newborns are especially vulnerable to hyperbilirubinemia-induced neurological damage, because in the earliest days of life, the still-developing liver is heavily exercised by the breakdown of fetal hemoglobin as it is replaced with adult hemoglobin and the blood–brain barrier is not as developed. Mildly elevated serum bilirubin levels are common in newborns, and neonatal jaundice is not unusual, but bilirubin levels must be carefully monitored in case they start to climb, in which case more aggressive therapy is needed, usually via light therapy but sometimes even via exchange transfusion.