60% of newborn babies born between 37 and 42 weeks gestation look a bit yellow (early onset neonatal jaundice) around about day 2 or 3 of life. Babies born earlier than 37 weeks are even more likely to be jaundiced.
So if it is that common why do we worry about it? The answer is that a tiny number of these babies are at risk of brain damage from extremely high levels of bilirubin and we want to make sure we catch these ones early enough to get the levels down before any damage occurs. To put things in perspective, there are about 5 cases of brain damage due to bilirubin toxicity a year in the UK and that is out of about 690,000 (2007 figures) babies born per year, at least 414,000 of whom would have been visibly jaundiced. So we do a huge amount of fairly disruptive testing for very little gain. However if your baby is one of the 5 a year it is devestating especially as the problem is potentially preventable.
The recent NICE guidelines on newborn jaundice say that any baby who the family or health professionals think is jaundiced must have their bilirubin level checked. Bilirubin is a breakdown product of the excess red blood cells that babies are born with and it is this that makes them look a bit yellow. Bilirubin can be measured in the skin painlessly with a small hand held device called a transcutaneous bilirubinometer. If your midwife does not have one of these or the reading on it is above a certain level then your baby will need a blood test. They may then need further tests to monitor the bilirubin level and check that your baby does not need treatment. Treatment in all but the most severe cases is phototherapy. The baby is put under or on a blue light or wrapped in a luminous blanket. The wavelengths of light in phototherapy break the bilirubin in the baby’s blood down into a soluble form which the baby can then safely wee out. Unless the bilirubin levels are very high you should be able to continue to breastfeed your baby while he or she is undergoing phototherapy.
NICE has a detailed parent information leaflet on jaundice available here.