When the colour changes, and when it settles
Many babies are born with very little pigment in the front of the iris, which is why their eyes can look very blue or grey in the first weeks. Over the first months, more pigment builds up and the eyes often darken: blue can become less blue, turn green or hazel, or turn completely brown.
The change is quickest early on. It slows down after about six months, but there can still be plenty left to come. For most children the colour has stopped changing by the end of the first year. For some it keeps shifting for several more years.
That is why one paediatrician, writing for the American Academy of Pediatrics, will not call a baby's eye colour until the child is at least a year old. If you want to know exactly how many months your baby is, the baby age calculator works it out.
Not every baby starts with blue eyes
Blue at birth is common, but it is not the rule. In a study of 192 healthy newborns at a children's hospital in California, 63% had brown eyes at birth, about 21% had blue eyes and about 6% had green or hazel eyes. Most of the rest were hard to judge from the photos.
So if your baby was born with brown eyes, there is nothing unusual about it. If they were born with blue or grey eyes, it is too early to tell where they will end up.
Where eye colour comes from
The colour of the iris depends on how much melanin, a brown pigment, sits in its front layers. Pigment cells that make only a little melanin give blue eyes, a bit more gives green or hazel, and a lot gives brown, the most common colour.
Blue eyes have no blue pigment at all. They look blue for the same reason the sky and water do: they scatter light so that more blue light bounces back out.
How much melanin the iris makes is set by genes from both biological parents. Two genes on chromosome 15, OCA2 and HERC2, have the biggest effect. MedlinePlus Genetics lists eight more with smaller roles, whose effects combine into the whole range of eye colours.
What the parents' eyes can tell you
The baby eye colour calculator above uses the classic two-gene model. One gene comes in a brown and a blue version, the other in a green and a blue version. Brown wins over green, and green wins over blue. A parent can carry a hidden lighter version without showing it, which is how two brown-eyed parents can have a blue-eyed child.
A few examples of what the model says:
- A brown-eyed and a blue-eyed parent: 50% brown, 25% green, 25% blue. The same American Academy of Pediatrics paediatrician puts it simply: the odds are about even.
- Two blue-eyed parents: blue in the model. In real life a blue-eyed child is very likely, but not certain.
Why it is only an estimate
The two-gene model is a simplification. Scientists once thought brown was simply dominant over blue, so two blue-eyed parents could never have a brown-eyed child. MedlinePlus Genetics calls that model too simple: it is uncommon, but blue-eyed parents can have brown-eyed children, because many genes work together.
The calculator also assumes that each parent carries every hidden version their eye colour allows, which is not true in every family. Take its percentages as a fun guide to what is likely, not a prediction for your baby.
Eye colour can even keep changing past the toddler years. In a long-running study of more than 1,500 twins in Louisville, Kentucky, most children had a stable eye colour by age 6. But the authors estimated that 10% to 15% of white participants kept seeing changes through their teens and into adulthood. Identical twins kept almost the same eye colour as each other, while fraternal twins grew less alike with age, which suggests that the tendency to change is partly genetic too.
When to ask your doctor
A baby's eyes changing colour in the first year is normal and expected. A few things are worth a check, without jumping to conclusions:
- Eyes of clearly different colours. If one eye is brown and the other blue, for example, bring it to your doctor's attention.
- A white glow in the pupil. The NHS advises an urgent appointment with your GP or an optician if you notice a white glow or white reflection in the centre of your child's eye, which can also show up in photos taken with a flash.
- Other changes. Eyes that point in different directions, or one iris that changes colour on its own, are also worth raising with your child's doctor or health visitor.
None of these is a diagnosis. They are simply things a doctor should see for themselves.