New parents frequently ask about their baby’s eye color. While the color at birth may be striking, it is rarely the final shade they will carry into adulthood. Infant eye color is not fixed and generally undergoes a period of gradual change over the first few months and years of life.
The Science Behind Infant Eye Color
Eye color is determined by the amount of a pigment called melanin present in the iris, the colored part of the eye. Specialized cells within the iris, known as melanocytes, are responsible for producing this melanin. The more active these cells are and the more melanin they secrete, the darker the eye color will be, with brown eyes having the highest amount of pigment.
Most babies are born with eyes that appear blue or gray because the melanocytes in their irises have not yet been fully activated. During gestation, there is little to no light exposure, which means the melanocytes have not begun their full production of melanin. Once a baby is born and regularly exposed to light, this process is stimulated, and the cells begin to produce the genetically determined amount of pigment.
The appearance of blue eyes is actually an optical effect, not the presence of blue pigment. When there is very little melanin in the front layer of the iris, light entering the eye is scattered back out, a phenomenon similar to what makes the sky look blue. As melanin production increases, this light scattering is reduced, and the eyes begin to take on their permanent color, which can range from green or hazel to dark brown.
The Typical Timeline for Color Stabilization
The initial color a baby is born with, whether it is blue, gray, or a dark brown, may not be the one that lasts. The earliest changes usually become noticeable around the 3 to 6-month mark as melanocytes ramp up production. During this time, the first subtle flecks of green, hazel, or brown may begin to appear in the iris.
The most dramatic shift in color often occurs between 6 and 9 months of age. By a child’s first birthday, the eye color is often close to its final shade, though the process is not yet complete. It is not uncommon for eye color to continue subtly darkening or shifting slightly throughout the toddler years.
Eye color is generally considered stabilized by the age of two or three years, when the melanocytes have produced their maximum amount of melanin. Minor changes can continue for a few more years, with stabilization almost always occurring by age six. Babies born with dark brown eyes often experience less change, as their melanocytes are highly active from birth.
How Genetics Determine the Final Shade
The final eye color is determined by a complex interplay of inherited genes, not a simple dominant and recessive pattern. Eye color is a polygenic trait, meaning multiple genes work together to control the amount and placement of melanin in the iris. While the old model suggested brown eyes were dominant over blue, modern science recognizes that the outcome is far more nuanced.
Two genes, OCA2 and HERC2, located on chromosome 15, play the largest role in determining eye color variation. The OCA2 gene provides instructions for the P protein, which is involved in melanin production and storage. The HERC2 gene acts as a regulatory switch, influencing how much the OCA2 gene is expressed, ultimately controlling the amount of pigment created.
The combination of variations in these and other genes dictates the final shade, making it challenging to predict with absolute certainty. For example, two parents with blue eyes are highly likely to have a child with blue eyes, but other outcomes are possible due to the involvement of multiple genes. Conversely, two brown-eyed parents can sometimes have a child with lighter eyes.