Bilirubin comes from red blood cells. Normally, the body destroys some red blood cells. When this happens, hemoglobin, the oxygen-carrying molecule within the cells, is released into the blood and converted to bilirubin. In the fetus, the placenta removes bilirubin from the fetal bloodstream. After the baby is born, the liver removes bilirubin from the blood, modifies it, and excretes it into the bile. The bile flows into the intestine so that the bilirubin is eliminated in the stool.
Many babies develop jaundice in the first few days of life that is typically worse at 2-4 days of age. This is known as physiological jaundice and typically goes away within two weeks. Mild jaundice can also result from breastfeeding. Severe jaundice in newborns results from medical conditions that cause an excessive turnover of red blood cells, including abnormal blood cell shapes and mismatch between maternal and fetal blood types. Certain medications and medical conditions can also slow down the process of removing bilirubin from the body and lead to jaundice. Some twins and babies who are small for their gestational age also have jaundice due to higher levels of red blood cells.
Icterus is the medical term for jaundice. Kernicterus is the term that refers to brain damage due to elevated bilirubin levels.