Bilirubin is a substance produced by the breakdown of old red blood cells and hemoglobin, the protein in red blood cells that carries oxygen from the lungs to the rest of the body. Bilirubin is removed from the body through the digestive system as part of the bile from the liver.

Normally, the body is constantly making new red blood cells and breaking down old ones. The amount of bilirubin in a person's blood is usually very low. Higher-than-normal amounts of bilirubin in the blood cause a person's skin and eyes to become yellow (jaundice). A blockage in the bile duct or certain medical conditions, such as liver disease, may cause bilirubin levels to rise to an unhealthy level.

Newborns often have high amounts of bilirubin in their blood. In most cases, the baby's system will become better able to eliminate the bilirubin within days. However, a baby may need medical attention if the yellowing appears to increase after the third day of life or has not decreased by the fifth day.