Jaundice is caused by a build up of bilirubin; a yellow substance that is produced when red blood cells, which carry oxygen around the body, are broken down!
When there is too much bilirubin in the blood, it is called hyperbilirubinemia. When the yellow coloured bilirubin is evident in the skin, eyes and other tissues, it is called jaundice.
Whilst in utero, the placenta deals with bilirubin, but when a baby is born their liver takes over this function. The liver processes bilirubin so it can leave the body via poo (the newborns first is called meconium – covered in more detail on week 33), and through urine. A newborn’s liver is not fully developed and can struggle to keep up with demand, removing bilirubin from the blood. Once the newborn reaches about 2 weeks old, the liver is more effective and often jaundice corrects itself, causing no harm.
Jaundice in newborns is common – 6 in every 10 babies develop jaundice. That jumps to 8 in every 10 for premature babies (born before 37 weeks). However, only 1 in 20 babies have bilirubin levels high enough to need treatment.
For more info on jaundice in newborns, check out this page from the NHS.