Jaundice is a yellowing of the baby's skin and eyes. It is due to the build up of a substance called 'bilirubin' in the baby's system. Bilirubin is the by-product of the normal cell breakdown of the baby's surplus red blood cells, in the first week or so after the birth. The red cells are called 'haemoglobin' and their main function is to carry oxygen.

Some of the haemoglobin becomes redundant after the birth, because the baby no longer needs to share oxygen with their mother. The extra haemoglobin is nature's way of allowing the baby to share oxygen with their mother while growing inside the uterus.

The bilirubin by-product (of the red cell breakdown) needs to be processed (or 'conjugated') by the baby's liver, before it can be excreted through the baby's bowel motions. About 50% to 70% of babies will process and excrete all the bilirubin in their system before it has a chance to accumulate. These babies will show no signs of yellowing. However, up to 30 to 50% of babies will temporarily accumulate varying levels of unconjugated bilirubin in their blood. This is because their liver cannot keep up with the amount of bilirubin that needs to be processed for excretion. The accumulation of unconjugated bilirubin gives the baby's skin and eyes a yellow discolouration, known as 'jaundice'.

Most babies with jaundice have only a slight yellow discoloration that does not cause them any health problems. Their liver eventually processes all the accumulated bilirubin within a week or so, without the level becoming very high. In some cases though, the levels of bilirubin can accumulate to high levels, usually requiring some form of treatment (mainly phototherapy). Caregivers will use various methods to monitor the levels of bilirubin in newborn babies, so they can intervene before they become too high.