Many newborn babies can have yellowish skin and eyes when they're only two to four days old. It's a medical condition called jaundice, and it can have lots of different causes, but usually their skin reaches its normal tone in just a week or two, after their organs are more developed. So what exactly causes jaundice, and why is it so common in newborn babies?

The word "jaundice" actually comes from the French word "jaune", meaning yellow. It's also sometimes known as "Ictarus", which comes from an ancient Greek word that referred to the medical condition and... Yellow bird? The stories say that they thought jaundice could be cured by staring at a yellow bird and magically transferring the color to it, which isn't a thing that works, in case you were wondering.

Jaundice actually occurs when your blood contains an excess of the molecule bilirubin. Bilirubin is a yellowish waste compound that's produced when old red blood cells are broken down. It's what gives bruises a yellowish hue and helps make your poop brown. Normally, bilirubin is filtered from your bloodstream by your liver, and then excreted through your intestines, but a newborn's liver is still developing, and can't remove bilirubin as quickly from their bloodstream, which turns their skin yellowish.

This mild buildup is actually pretty common in babies, it's referred to as "normal jaundice", but there are a few other forms. 

Jaundice of prematurity is seen in premature babies because their bodies are even less equipped to get rid of extra bilirubin. 

Babies can develop breast-feeding jaundice if they're not getting enough breast milk from their mothers. In that case, they don't have as many bowel movements, and they don't excrete as much bilirubin. 

Breast milk jaundice is less common and is caused by substances in the mother's breast milk that can interfere with the liver's bilirubin processing. 

Then there's Blood Group Incompatibility, which can happen in some cases when the mother and the baby have different blood types. This means the mother's body could make antibodies which attack her baby's red blood cells, resulting in an excess buildup of bilirubin in the baby's bloodstream. 

Most of these forms of infant jaundice typically disappear without any extra treatment in a couple of weeks as the newborn's body matures. But if a baby develops more severe jaundice, or has other sickly symptoms, they should be taken to a hospital where doctors can lower the level of bilirubin in their blood. These treatments range from something called phototherapy, where special blue-spectrum light helps break down bilirubin in the baby's body, to a blood transfusion that gives them more healthy red blood cells.

One thing they don't have babies do, though, is stare at a yellow bird.

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