The human eye does not remain the same size from infancy to adulthood; instead, it undergoes significant growth, particularly in its front-to-back dimension. This growth is a precisely regulated biological process that ensures the eye can focus light accurately onto the retina. The eye grows substantially from about 16.5 millimeters in diameter at birth to approximately 24 millimeters in adulthood. This represents an increase of nearly 50% in length.
The Major Phases of Eye Growth
The development of the eye follows a distinct timeline with two major phases of rapid growth. The first and most intense growth spurt occurs during the first two years of life. During this time, the eye grows rapidly from being roughly two-thirds its final size to nearing its adult dimensions.
Following this initial burst, a slower, secondary phase of eye growth takes place. This phase continues steadily until approximately puberty or the early teenage years, often stabilizing around age 20 or 21. The eye’s growth rate slows down dramatically after early childhood.
Anatomy Behind the Size Increase
The overall size of the eyeball is primarily determined by its axial length, which is the measurement from the front of the cornea to the back wall of the retina. An adult eye measures about 24 millimeters in this direction. The physical expansion of the eye is mainly driven by the growth of the sclera, the tough, white outer layer that provides the eye’s structure.
This outward expansion increases the volume of the vitreous humor, the clear, gel-like substance that fills the large space at the back of the eye. In contrast, the lens and the cornea, the clear structures at the front of the eye, are already close to their adult size at birth. These frontal components change minimally in overall volume, primarily adjusting their shape or curvature to maintain focusing power as the axial length increases.
Why Eyes Seem Large at Birth
The perception that a baby’s eyes are already full-sized is due to an optical illusion created by facial proportions. At birth, the cornea and the lens are already near their adult diameter, meaning the visible parts of the eye are disproportionately large compared to the rest of the face. The diameter of the cornea, for instance, reaches its full width around three months of age.
The skull and facial bones continue to expand for years. Because the eyes are one of the fastest-growing organs during the prenatal period, they appear to take up a much larger percentage of the infant’s face. This proportional difference creates the visual effect of a child having comparatively large eyes in a small face.
How Growth Impacts Vision
The process of eye growth is highly regulated and is known as emmetropization, a mechanism that aims to achieve clear vision without the need for corrective lenses. This biological feedback loop ensures that the eye’s length precisely matches the focusing power of the cornea and lens. The goal is for the focal point of incoming light to land directly on the retina.
If the axial length grows too long relative to the fixed power of the front structures, the focal point falls in front of the retina, resulting in nearsightedness, or myopia. Conversely, if the eye stops growing too short, the light focuses behind the retina, causing farsightedness, or hyperopia. The increasing prevalence of myopia in children is often linked to the eye growing too quickly or too long, pushing emmetropization off balance.