question from @vintage_rose06 via twitter
This is an often quoted fact to new mothers. A quick bit of reading unearths plenty of people confirming this. There are also plenty of people whose experiences disagree with it. It seems to be that this phenomenon is only true of Caucasian babies – babies of Asian or black ethnicity typically have brown eyes which don’t change colour. Caucasian babies are commonly born with blue eyes, which may then become green/brown/etc. after about 6-9 months (or remain blue).
The iris is the coloured part of the eye and colouration is caused by a protein called melanin. It is also the same substance that causes skin colouration and tanning. Melanin in the eye causes a person’s irises to be coloured, usually either green or brown. People with blue eyes have genes which mean they have only low levels of melanin being deposited in the iris. Such irises are actually clear – but appear blue because of the way light bounces off it. For the physics-savvy: one interesting property of the eye result in short wavelength light (i.e. blue) being reflected back out, with longer wavelength light (e.g. reds) being absorbed by the underlying layer of dark cells.
A new-born baby is not likely to have attained full melanin production, which increases over the first year of life. The iris will have a low concentration of melanin and so appear blue at birth and then as the baby develops, melanin production would increase and the eye may change colour. I would suggest that the higher levels of melanin in black and Asian ethnicities mean that this effect is less noticeable as the iris will already have a fairly high level of melanin in it. A build up of melanin over time does, however, explain the lighter than expected skin colour sometimes seen in the children of darker skinned parents – which then darkens as melanin production increases over the first year.
Answered by James Crewdson