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 that it can be passed out of the body in stools (faeces).

However, if there is too much bilirubin in the blood, or if the liver cannot get rid of it, excess bilirubin will cause jaundice.

Jaundice in babies

Jaundice is common in newborn babies because babies have a high level of red blood cells in their blood and these are broken down and replaced frequently. The liver in newborn babies is also not yet fully developed, so it is less effective at processing the bilirubin and removing it from the blood.

This means that the level of bilirubin in babies can be about twice as high as in adults.

By the time a baby is about two weeks old, they are producing less bilirubin and their liver is more effective at removing it from the body, which means the jaundice often corrects itself by this point without causing harm.

Breastfeeding

Breastfeeding your baby can increase their chances of developing jaundice, but there is no need to stop breastfeeding your baby if they have jaundice as the symptoms will normally pass in a few weeks and 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 require 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:

• an underactive thyroid gland (hypothyroidism) – where the thyroid gland does not 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
• an inherited enzyme deficiency (enzymes are proteins that speed up and control chemical reactions in the body) – such as a deficiency of glucose-6-phosphate-dehydrogenase (G-6-PD), an enzyme that helps red blood cells function
• a urinary tract infection 
• Crigler-Najjar syndrome – an inherited condition that adversely affects the enzyme responsible for processing bilirubin, leading to a build-up of bilirubin
• an obstruction or defect in the biliary system – the biliary system is made up of the bile ducts and gall bladder, and it creates and transports bile (a fluid used to help digest fatty foods)

Blood

Blood supplies oxygen to the body and removes carbon dioxide. It is pumped around the body by the heart.

Intestines

The intestines are the part of the digestive system between the stomach and the anus that digests and absorbs food and liquid.

Liver

The liver is the largest organ in the body. Its main jobs are to secrete bile (to help digestion), detoxify the blood and change food into energy.

Oxygen

Oxygen is an odourless, colourless gas that makes up about 20% of the air we breathe.