When Do Babies' Eyes Change Colour? The Science of Melanin and Iris Development

Many parents are surprised to discover that the eye colour their baby is born with may not be the eye colour they end up with. The biology behind eye colour development is genuinely interesting and explains why your blue-eyed newborn might well become a brown-eyed toddler — and when you can expect things to settle.

Why Are Many Babies Born With Blue or Grey Eyes?

Eye colour is determined by the amount and type of melanin — a natural pigment — in the iris (the coloured part of the eye). More melanin produces darker colours; less produces lighter ones.

The melanin in the iris comes from cells called melanocytes. These cells are present in the eye from birth, but they have not yet been fully activated. Melanocyte activity is stimulated by exposure to light. In the months after birth, as a baby's eyes are increasingly exposed to light, the melanocytes in the iris begin to produce more melanin — and this is what causes the colour to deepen.

Babies of all ethnicities are born with relatively low melanin in their irises. In babies with lighter genetic colouring, this results in the characteristic blue-grey of many newborns' eyes. In babies with darker genetic colouring, eyes are often a darker grey or brown at birth, since some melanin production is already underway.

When Does Eye Colour Change?

The most significant changes in eye colour typically happen in the first six to twelve months of life. By six months, most babies' eyes have begun to show their more permanent colour. However, the process can continue:

• In babies whose eyes are darkening, change is usually visible by three to six months
• Final, stable eye colour is typically set by twelve to eighteen months of age
• In some children, subtle shifts can continue until around three years of age

The change, when it happens, is usually gradual — a slow deepening or shift in hue rather than an overnight transformation.

Can Blue Eyes Turn Brown?

Yes. This is one of the more common eye colour journeys. A baby born with blue eyes can develop brown, hazel, or green eyes as melanin production increases. The reverse — brown eyes turning blue — does not happen, because brown eyes already contain substantial melanin. Once melanin is produced in the iris, it does not significantly reduce.

So: eyes can darken, but they cannot lighten significantly after the newborn stage.

What Does Eye Colour Tell Us About Genetics?

Eye colour is a complex genetic trait controlled by multiple genes — primarily OCA2 and HERC2, which regulate melanin production, but also a number of other genes that modify the result. The old model of eye colour genetics — two brown-eyed parents always produce brown-eyed children, blue eyes are recessive — is an oversimplification.

In reality:

• Two blue-eyed parents can have a brown-eyed child (though this is relatively uncommon)
• Two brown-eyed parents can have a blue-eyed, green-eyed, or hazel-eyed child, depending on the genetic variants they carry
• Eye colour prediction, even with genetic testing, is probabilistic rather than certain for intermediate colours

The OCA2 gene, on chromosome 15, plays the largest single role. The HERC2 gene acts as a "switch" that regulates OCA2 activity. Variations in HERC2 largely determine whether significant amounts of brown melanin are produced.

Green eyes result from a relatively small amount of melanin combined with the optical effect of light scattering (Rayleigh scattering) in the iris. Green is among the rarest eye colours globally — approximately 2 per cent of the world population. It is most common in people of European descent, particularly in Ireland, Scotland, and Northern Europe.

Hazel eyes are a mix of melanin and the same light-scattering effects, producing an appearance that can shift between brown, green, and gold depending on lighting conditions.

What to Expect With Your Baby

If your baby was born with blue or grey eyes and neither parent has blue eyes, do not necessarily expect them to stay blue — the genetic contribution of both parents' full genetic make-up will determine the outcome, not just their own eye colour.

If your baby was born with very dark brown eyes, the colour is unlikely to change significantly.

If you are somewhere in between — grey, blue-grey, or a pale brown — watch over the first year. The most informative indicator will simply be the colour at their first birthday, by which point most significant changes have already occurred.

A Note on Heterochromia

Some babies are born with — or develop — different coloured irises, or patches of a different colour within a single iris. This is called heterochromia and is usually entirely benign, caused by an uneven distribution of melanin between the irises during development.

In rare cases, acquired heterochromia (a change in eye colour in one eye after birth) can be associated with underlying conditions such as Horner syndrome, uveitis, or other eye or systemic conditions. If your baby's irises appear different from each other and this was not present at birth, mention it to your GP or health visitor.