A newborn’s eyes often appear light, typically blue or gray, but this initial shade is temporary. This gradual transformation is a natural part of infant development, signaling that the final eye color is not fixed at birth. The change is governed by the activation of specialized cells and the genetic instructions passed down from both parents.
The Typical Timeline for Eye Color Stabilization
The most noticeable changes in eye color typically begin within the first six months of life. Initial shifts often start between three and six months of age, as pigment production increases and the iris may show flecks of a deeper color.
The color is generally considered stable around the baby’s first birthday. The American Academy of Ophthalmology suggests that most babies settle into their lifelong eye color by about nine months old. While the majority of the change is complete sooner, subtle shifts may proceed until a child is three years old.
This timeline primarily applies to babies born with lighter eye colors, such as blue, gray, or hazel. Infants born with dark brown eyes already have a high level of pigment in their irises, meaning their color is often stable from day one.
How Melanin Production Determines Eye Color
The biological mechanism behind this change centers on a pigment called melanin, the same substance that gives color to skin and hair. Eye color is determined by the amount of melanin present in the stroma, which is the front layer of the iris. Eyes that appear blue have very little melanin in this layer, while brown eyes contain a significant amount.
Most babies are born with light eyes because the specialized cells responsible for producing melanin, called melanocytes, have not been fully activated. The fetus develops in a dark environment without much light exposure, so the melanocytes remain dormant. After birth, exposure to light acts as the trigger, stimulating these cells to begin producing melanin.
The slow, steady increase in melanin production is what causes the eye color to gradually darken, perhaps moving from blue to green, hazel, or brown. The color change will only go from light to dark because the body only adds pigment over time, it does not remove it. The final color depends on the genetic potential for the melanocytes to produce and store this pigment.
The Basics of Eye Color Genetics
While the timing of the change is driven by light exposure and pigment activation, the final color is dictated by the baby’s genetic inheritance. Eye color is a polygenic trait, meaning it is influenced by multiple genes, not just a single one, which makes the exact outcome complex to predict. Scientists have identified up to 16 different genes that play a role, with two major genes on chromosome 15 having the largest influence.
The genes inherited from the parents determine the maximum potential amount of melanin the melanocytes can produce. For instance, the allele for brown eyes is typically dominant over the alleles for green or blue eyes. This is why two brown-eyed parents have a high probability of having a brown-eyed child, though a blue-eyed child is still possible if both parents carry the recessive blue-eye gene.
Understanding the parents’ and grandparents’ eye colors provides the best clue about the baby’s eventual hue, but the multi-gene nature of the trait means there can be unexpected variations. In rare instances, a baby may inherit a condition called heterochromia, resulting in two differently colored eyes. Ultimately, genetics establishes the blueprint, and melanin production executes the final shade.