It’s a good idea to hold off on buying the adorable outfit that matches your baby’s eye color — at least until your little one reaches their first birthday.

That’s because the eyes you gaze into at birth may look a bit different at 3, 6, 9, and even 12 months of age.

So before you get too attached to those 6-month-old green eyes, just know that some babies will experience changes up to 1 year of age. Some little ones’ eye color even continues to change hues until they’re 3 years old.

Your baby’s first birthday is a significant milestone, especially if they get to dive into a cake for the first time. But it’s also about the age you can safely say your baby’s eye color is set.

“Typically, a baby’s eyes can change color during the first year of life,” says Benjamin Bert, MD, an ophthalmologist at Memorial Care Orange Coast Medical Center.

However, Daniel Ganjian, MD, a pediatrician at Providence Saint John’s Health Center, says the most significant changes in color occur between 3 and 6 months.

But the hue you see at 6 months may still be a work in progress — which means you should wait a few months (or more) before filling in the eye color section of the baby book.

Although you can’t predict the exact age your baby’s eye color will be permanent, the American Academy of Ophthalmology (AAO) says most babies have the eye color that will last their lifetime by the time they’re about 9 months old. However, some can take up to 3 years to settle into a permanent eye color.

And when it comes to the color your baby’s eyes will take on, the odds are stacked in favor of brown eyes. The AAO says that half of all people in the United States have brown eyes.

More specifically, a 2016 study involving 192 newborns found that the birth prevalence of iris color was:

  • 63% brown
  • 20.8% blue
  • 5.7% green/hazel
  • 9.9% indeterminate
  • 0.5% partial heterochromia (a variation in coloration)

The researchers also found that there were significantly more white/Caucasian infants with blue eyes and more Asian, Native Hawaiian/Pacific Islander, and Black/African American infants with brown eyes.

Now that you have a better understanding of when your baby’s eyes may change color (and become permanent), you might be wondering what’s going on behind the scenes to make this transformation occur.

Melanin, a type of pigment that contributes to your hair and skin color, also plays a role in iris color.

While some baby’s eyes are blue or gray at birth, as the study above noted, many are brown from the start.

As melanocytes in the iris respond to light and secrete melanin, the American Academy of Pediatrics (AAP) says the color of a baby’s irises will begin to change.

Eyes that are a darker shade from birth tend to stay dark, while some eyes that began a lighter shade will also darken as melanin production increases.

This typically occurs over their first year of life, with the color change slowing down after 6 months. A small amount of melanin results in blue eyes, but increase the secretion and baby may end up with green or hazel eyes.

If your baby has brown eyes, you can thank the hardworking melanocytes for secreting a lot of melanin to produce a darker color.

“It’s the melanin granules deposited in our iris that gives us our eye color,” says Bert. And the more melanin you have, the darker your eyes become.

“The pigment is actually all brown in appearance, but the amount present in the iris can determine if you have blue, green, hazel, or brown eyes,” he explains.

That said, Bert points out that even the possibility of the eyes changing color depends on the amount of pigment they begin with.

You can thank genetics for your baby’s eye color. That is, the genetics that both parents contribute.

But before you go high fiving yourself for passing on your brown eyes, you should know that it’s not just one gene that determines your little one’s eye color. It’s many genes acting in collaboration.

In fact, the AAO says as many as 16 different genes could be involved, with the two most common genes being OCA2 and HERC2. The other genes can pair with these two genes and create a continuum of eye colors in different people, according to the Genetics Home Reference.

Although uncommon, that’s why your children may have blue eyes even though you and your partner have brown.

More likely, two blue-eyed parents will have a child with blue eyes, just like two brown-eyed parents will likely have a brown-eyed child.

But if both parents have brown eyes, and a grandparent has blue eyes, you increase the odds of having a blue-eyed baby, according to the AAP. If one parent has blue eyes and the other has brown, it’s a gamble as to the color of baby’s eyes.

“Some eye disease can affect color if they involve the iris, which is the muscular ring around the pupil that controls pupil contracting and dilating when we go from [a] dark to light place, and vice versa,” says Katherine Williamson, MD, FAAP.

Examples of these eye diseases include:

  • albinism, where the eyes, skin, or hair have little or no color
  • aniridia, the complete or partial absence of the iris, so you’ll see little or no eye color and, instead, a large or misshapen pupil

Other eye diseases aren’t visible, however, like color blindness or glaucoma.

Heterochromia, which is characterized by irises that don’t match in color in the same individual, can happen:

  • at birth due to genetics
  • as a result of another condition
  • due to a problem during eye development
  • because of injury or trauma to the eye

While all babies develop at different rates, experts say if you notice two different eye colors or a lightening of eye color by 6 or 7 months of age, it’s a good idea to contact your pediatrician.

Your baby will experience a lot of changes during their first year of life. Some of these changes you may have a say in, while others are entirely out of your control.

Besides contributing your genes, there’s not much you can do to influence the color of your baby’s eyes.

So, while you may be rooting for “baby blues” or a “brown-eyed girl,” it’s best not to get too attached to your little one’s eye color until after their first birthday.