Causes of jaundice in newborn babies 
Jaundice is caused by too much bilirubin in the blood. This is known as hyperbilirubinaemia.
Bilirubin is a yellow substance produced when red blood cells, which carry oxygen around the body, are broken down.
The bilirubin travels in the bloodstream to the liver. The liver changes the form of the bilirubin so it can be passed out of the body in poo. 
However, if there’s too much bilirubin in the blood or the liver can’t get rid of it, excess bilirubin causes jaundice.
Jaundice in babies
Jaundice is common in newborn babies because babies have a high level of red blood cells in their blood, which are broken down and replaced frequently. 
The liver in newborn babies is also not yet fully developed, so it’s less effective at processing the bilirubin and removing it from the blood.
This means the level of bilirubin in babies can be about twice as high as in adults.
By the time a baby is around two weeks old, they’re producing less bilirubin and their liver is more effective at removing it from the body. This means the jaundice often corrects itself by this point without causing any harm.
Breastfeeding
Breastfeeding your baby can increase their chances of developing jaundice. However, there’s no need to stop breastfeeding your baby if they have jaundice as the symptoms normally pass in a few weeks. 
The benefits of breastfeeding outweigh any potential risks associated with the condition.
If your baby needs to be treated for jaundice, he or she may need extra fluids and more frequent feeds during treatment. See treating newborn jaundice for more information.
The reason why breastfed babies are more likely to develop jaundice is unclear, although a number of theories have been suggested. For example, it may be that breast milk contains certain substances that reduce the ability of the liver to process bilirubin.
Newborn jaundice thought to be linked to breastfeeding is sometimes called breast milk jaundice.
Underlying health conditions
In some cases, jaundice may be the result of another health problem. This is sometimes called pathological jaundice.
Some causes of pathological jaundice include:

• hypothyroidism – where the thyroid gland doesn’t produce enough hormones 
• blood group incompatibility – when the mother and baby have different blood types, and these are mixed during the pregnancy or the birth 
• rhesus factor disease – a condition that can occur if the mother has rhesus-negative blood and the baby has rhesus-positive blood 
• a urinary tract infection
• Crigler-Najjar syndrome – an inherited condition that affects the enzyme responsible for processing bilirubin 
• a blockage or problem in the bile ducts and gallbladder – these create and transport bile, a fluid used to help digest fatty foods 

An inherited enzyme deficiency known as glucose 6 phosphate dehydrogenase (G6PD) could also lead to jaundice or kernicterus. 
If you have a family history of G6PD, it’s important to let your midwife, GP or paediatrician know and your baby’s jaundice symptoms are closely monitored.