Eye color isn\u2019t fixed at birth. Instead, the shade can change through biological processes that involve melanin, sun exposure, and genetics, said Dr. Rebecca Chasnovitz<\/a>, an associate professor of pediatrics at the University of North Carolina at Chapel Hill\u2019s School of Medicine.<\/p>\n \u201cNot all babies\u2019 eyes change color,\u201d Chasnovitz said. \u201cBut if babies are born with light eyes, then sometimes their eyes will slowly darken over the first year.\u201d\u00a0<\/p>\n How do babies\u2019 eyes change color?\u00a0<\/p>\n Specialized cells in a baby\u2019s eyes, called melanocytes, produce a protein called melanin\u2014which darkens the skin, but also darkens our irises, the colored part of the eye. According to the National Library of Medicine<\/a>, the more melanin, or pigment, we have in the front part of our iris, the darker our eyes will be. People with brown eyes, the most common shade in the world<\/a>, have more melanin than those with lighter shades.<\/p>\n People with blue eyes, for example, have no melanin in the front part of their iris, according to the American Academy of Ophthalmology<\/a>. People with green or hazel eyes have light brown pigment in their eyes.\u00a0<\/p>\n As babies are exposed to sunlight, those specialized cells\u2014the melanocytes\u2014become more active, producing more melanin, Chasnovitz said. Normal sun exposure is all it takes to trigger the changes, such as regular stroller walks through the neighborhood or ambient light in the home.\u00a0<\/p>\n Genetics add another layer to the process, determining how much melanin an individual\u2019s iris will produce. But, unlike simple inheritance patterns, eye color isn\u2019t determined by a single gene. Multiple genetic markers contribute to the final shade, making it not always easy to predict the final outcome.<\/p>\n In other words, if both of a baby\u2019s parents have brown eyes, that doesn\u2019t mean their offspring will also have brown eyes. \u201cTwo people with lighter eyes are more likely to have a baby with lighter eyes. Two people with darker eyes are likely to have a darker-eyed baby. But if a grandparent has light eyes, they might end up with light eyes,\u201d Chasnovitz said. \u201cIf you have a lighter eyed parent and a darker eyed parent, it\u2019s kind of a toss-up what it\u2019s going to be.\u201d\u00a0<\/p>\n A small study in 2016 of 192 babies<\/a> at Stanford University School of Medicine found that about 63 percent were born with brown eyes, about 21 percent with blue, 6 percent born with hazel or green eyes, and the remaining 10 percent could not be placed into one color category.\u00a0<\/p>\n When to pay attention to eye color<\/p>\n Parents typically start to see some changes in their child\u2019s eye color during their first six months, and the transition typically continues until the first birthday. \u201cThey\u2019ll look a little muddier if they\u2019re going to be darkening,\u201d Chasnovitz said.<\/p>\n There are rare exceptions where parents should take note of eye color, Chasnovitz said. For example, having one blue eye and one darker eye could indicate a genetic syndrome called Waardenburg syndrome<\/a>, which requires medical attention.\u00a0<\/p>\n Cross eyes or wandering eyes also aren\u2019t unusual in newborns, she said. By four to six months, however, if the issue hasn\u2019t resolved, she\u2019ll typically recommend going to an eye specialist.\u00a0<\/p>\n As Chasnovitz notes, \u201cEyesight is the least developed of all the senses when babies are born.\u201d\u00a0<\/p>\n This story is part of Popular Science\u2019s\u00a0Ask Us Anything series<\/a>,\u00a0where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. 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