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.

But if there's too much bilirubin in the blood or the liver cannot get rid of it, the excess bilirubin causes jaundice.

Jaundice is common in newborn babies because babies have a high number of red blood cells in their blood, which are broken down and replaced frequently.

A newborn baby's liver is not 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 is much higher than in adults.

By the time a baby is around 2 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 your baby can increase their chances of developing jaundice.

But there's no need to stop breastfeeding your baby if they have jaundice, because the symptoms normally pass in a few weeks.

Some breastfed babies can have jaundice for as long as 12 weeks, but it's important that this is checked by a health visitor or GP so other more serious causes of jaundice can be ruled out.

The benefits of breastfeeding outweigh any potential risks associated with the condition.

If your baby needs to be treated for jaundice, they may need extra fluids and more frequent feeds during treatment.

See treating newborn jaundice for more information.

It's unclear why breastfed babies are more likely to develop jaundice, but 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.

Sometimes jaundice may be caused by another health problem. This is known as 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, which are mixed during the pregnancy or the birth)
• rhesus disease (a condition that can occur if the mother has rhesus-negative blood and the baby has rhesus-positive blood)
• a urinary tract infection (UTI)
• Crigler-Najjar syndrome (an inherited condition that affects the enzyme responsible for processing bilirubin)
• a blockage or problem in the bile ducts and gallbladder (the gallbladder stores bile, which is transported by the bile ducts to the gut)

An inherited enzyme deficiency known as glucose 6 phosphate dehydrogenase (G6PD) could also lead to jaundice or kernicterus.

It's important to let your midwife, GP or paediatrician know if you have a family history of G6PD. Your baby's jaundice symptoms will need to be closely monitored.