Bilirubin is a yellow waste product resulting from the normal breakdown of red blood cells. The liver removes bilirubin from the bloodstream and sends it to the intestinal tract, where it makes its grand exit out of the body and into a diaper. Preemies, however, face two major problems when it comes to disposing of bilirubin: Their livers can't handle the bilirubin fast enough, and they may eat less, meaning they have fewer bowel movements. That means even the bilirubin that was filtered from the blood may not have anywhere to go.
Too much bilirubin, also known as hyperbilirubinemia, results in jaundice, which yellows the baby's skin and whites of the eyes. Signs of jaundice will probably begin appearing between the second and fourth day of life, and they can last for a few weeks in preemies. In rare cases, extremely high levels of bilirubin can lead to brain damage. Usually, however, doctors can treat the condition with light therapy. The baby will spend some time under special lights wearing just a diaper and some protective eyewear. Doctors will monitor a baby's bilirubin counts until they reach healthy levels.