The eyes begin forming remarkably early in pregnancy. The first visible structures appear around week 4, when two small pouches push outward from the developing brain. From that point, eye development unfolds in stages across the entire pregnancy, with the retina still maturing for months after birth.
Week 4: The Eyes Start as Brain Outgrowths
Eye formation begins when the neural tube (the structure that becomes the brain and spinal cord) closes during the fourth week of pregnancy. As it seals, two grooves on either side of the forebrain transform into balloon-like pouches called optic vesicles. These are literally outgrowths of the brain, which is why the eyes and brain share so many biological connections throughout life.
By the end of week 4, each optic vesicle has moved close enough to the surface of the embryo’s face to make contact with the skin-like tissue there. That contact triggers the overlying tissue to thicken into a lens placode, the earliest precursor of the lens. At this point the embryo is only about 4 to 5 millimeters long.
Weeks 5 Through 8: Core Structures Take Shape
Development moves quickly during weeks 5 and 6. Around day 32, the lens placode folds inward to form a small depression called the lens pit. By roughly day 33, that pit pinches off completely to become a sealed bubble, the lens vesicle, which will eventually become the transparent lens of the eye. At the same time, the optic vesicle folds in on itself to create a double-walled cup shape. The inner wall of this cup becomes the light-sensing retina; the outer wall becomes the pigmented layer behind it.
By the end of the first trimester (around week 13), eyelid folds have formed and fused shut over the developing eyes. They will stay sealed for months, protecting the delicate structures underneath as they continue to mature. This first trimester is also the most sensitive window for congenital eye defects. Because the major structures are actively forming, any significant disruption during this period carries the highest risk of conditions like abnormally small eyes or missing eye tissue.
Weeks 10 Through 21: The Retina Builds Its Layers
The retina, the tissue at the back of the eye responsible for detecting light, develops in a precise sequence over the second trimester. Cone cells, which handle color vision, begin appearing around week 10. Rod cells, which detect dim light, follow from about week 12 onward. These photoreceptor cells continue organizing themselves into orderly columns through about week 20, and by week 21 the photoreceptor layer is well defined.
The surrounding retinal layers develop on a similar schedule. By around week 19, the major cell layers of the retina become distinct from one another. The supportive membrane behind the retina is fully formed by week 20. This mid-pregnancy period, roughly weeks 19 through 21, is a defining window when the retina transitions from a loosely organized collection of cells into a recognizably layered structure.
Even so, the retina is far from finished. The fovea, the tiny central spot responsible for sharp central vision, doesn’t start forming its characteristic pit until around week 33. And the outermost segments of foveal photoreceptors continue maturing until about 5 months after birth. Full visual sharpness takes years to develop.
What Happens at the Anatomy Scan
The routine mid-trimester ultrasound, typically performed between 18 and 24 weeks, includes a check of the baby’s eyes. The sonographer looks for the presence of both eye sockets, confirms the eyeballs are visible within them, and checks that the eyes are normally positioned and spaced apart on the face. This scan can flag structural abnormalities like widely or narrowly spaced eyes, which sometimes signal underlying genetic conditions. Eye movements can actually be seen on ultrasound as early as 14 to 15 weeks, though that’s not a standard part of the scan.
Weeks 26 Through 31: Eyes Open and Respond to Light
The fused eyelids begin to separate around week 26, with the eyes partially open. By week 28, the eyes are fully open. This is also when the fetus starts responding to its visual environment in a meaningful way. By week 31, the pupils can constrict and dilate, and the fetus can detect light that passes through the abdominal wall. Research using ultrasound has confirmed that from 31 weeks onward, the fetal visual system is capable of directed vision when enough light is available.
The blink reflex also develops during this period. Premature infants born after about 25 weeks of gestational age show a measurable blink response, though the reflex continues to speed up and mature through week 37 and beyond.
Eye Color Develops Mostly After Birth
One thing the eyes don’t settle during pregnancy is their final color. The iris, the colored ring of muscle around the pupil, contains pigment-producing cells, but these cells need light exposure to generate their full amount of pigment. In the dim environment of the womb, very little pigment accumulates. That’s why many newborns have light blue-gray eyes regardless of their eventual eye color.
Once a baby is born and exposed to light, pigment production ramps up. Eye color typically starts shifting between 3 and 9 months of age, often noticeably around 6 months. The process can take up to three years to fully settle. Darker eye colors like brown reflect higher amounts of pigment, while lighter colors like blue result from less pigment in the iris.
Nutrients That Support Eye Development
Several nutrients play specific roles in building the fetal visual system. Vitamin A is essential for the formation of the retina’s light-sensing cells. DHA, an omega-3 fatty acid concentrated in fish, is a major structural component of retinal cell membranes. And lutein, a carotenoid found in leafy greens and eggs, accumulates preferentially in the developing eye and brain. Lutein becomes detectable in the fetal macula around week 33, right when the foveal pit begins forming, and continues building up through early childhood. Research in primates has shown that diets lacking lutein during prenatal development affect the structural composition of the retina, suggesting it plays a functional role rather than simply being stored there. Adequate intake of all three nutrients throughout pregnancy supports the long arc of retinal development that stretches from the first trimester well into infancy.