Long before your baby says mamá, agua, ball, or more, their brain is already doing an extraordinary amount of language learning.
In fact, it starts before birth.
On this episode of Bilingüe, Birdsong founder Ashley Rivera speaks with Dr. Krista Byers-Heinlein, Professor of Psychology at Concordia University, Concordia University Research Chair in Bilingualism and Open Science, and Director of the Concordia Infant Research Lab.
Dr. Byers-Heinlein has spent more than 15 years studying babies and children growing up with multiple languages. Her research asks questions bilingual parents think about all the time:
Can my baby tell their two languages apart? How much exposure is enough? Will two languages delay their first words? Does switching between languages confuse them? And what can I actually do at home to help?
Here's what the science tells us.
Language learning starts before birth
Babies don't suddenly begin learning language when they babble or say their first word.
During the last trimester of pregnancy, the auditory system is already developed enough for babies to listen to the sounds around them — particularly their mother's voice.
Dr. Byers-Heinlein's research has found evidence that this prenatal exposure matters.
When researchers studied newborn babies just one to three days after birth, babies who had been exposed to two languages prenatally showed interest in both languages.
Even more importantly, they could distinguish between them.
That means hearing two languages wasn't creating confusion. Their brains were already detecting that there were two different linguistic systems around them.
The takeaway for parents isn't that bilingual exposure has to begin during pregnancy. It doesn't.
It's something much more reassuring:
There is no age that's "too young" for your baby to hear two languages.
Before they can talk, babies are learning constantly
During those quiet newborn months when it can feel like you're talking to someone who isn't talking back, an enormous amount is happening behind the scenes.
Babies begin paying attention to:
- the rhythm of language
- speech sounds
- syllable patterns
- familiar voices
- facial expressions
- the movement of a speaker's mouth
And babies become increasingly tuned into the languages they hear regularly.
So talking to your newborn while you're changing a diaper, singing during bath time, or saying "Vamos a comer" before lunch isn't wasted because they can't respond yet.
They're listening.
Why "parentese" gets babies' attention
You probably already do this without thinking about it.
When adults talk to babies, their voices often become slightly higher, slower and more melodic:
"Hoooola, mi amor…"
Researchers call this infant-directed speech, sometimes called parentese.
Dr. Byers-Heinlein explains that babies are particularly interested in this style of speech. It captures their attention — and surprisingly, research suggests their preference for it can actually become stronger as they grow.
Bilingual babies respond to infant-directed speech too.
And their response reveals something fascinating: babies pay more attention to languages that are familiar to them.
Their brains are already learning, in a sense, "These are my languages. These are the ones I should pay attention to."
Babies learn language best from real people
This may be one of the most actionable lessons from the entire conversation.
A recording of someone speaking Spanish is not equivalent to a person interacting with your baby in Spanish.
Research has consistently found that babies learn better from live, responsive human interaction.
Part of the difference is attention. Babies pay considerably more attention to a real person than to prerecorded speech.
But there's another important ingredient: contingency.
When your baby looks, points, babbles or reacts, you respond.
That back-and-forth interaction gives language meaning.
Interestingly, video calls may offer some of that same benefit because another person can respond to the baby in real time.
So that FaceTime call with Abuela?
It isn't simply maintaining a family relationship. It can also become part of your child's language environment.
How much exposure does a bilingual baby need?
This is one of the questions bilingual parents ask most.
Dr. Byers-Heinlein recommends beginning with what she calls a "language scan."
Instead of asking, "Do I speak enough Spanish?", walk through your child's actual week.
Who is with them when they wake up?
What language does that person speak?
What language is daycare?
What happens after school?
What about weekends?
Once parents carefully work through a typical day, research suggests they're actually quite good at estimating the percentage of time their children hear each language.
As a rough rule of thumb, Dr. Byers-Heinlein points to approximately 20–25% of waking hours as a minimum exposure level that may support learning a language.
But she is careful to emphasize that this isn't a magic threshold.
Every child is different. The quality of the interaction matters. The people speaking the language matter. Community exposure matters. And a child's language environment changes continually as childcare, school and family routines change.
The bigger lesson?
Exposure adds up.
And for a heritage or minority language that your child won't hear nearly as often outside the home, it can be helpful to intentionally create more opportunities for it.
That might mean a Spanish-speaking babysitter, grandparents, a weekly playdate, community activities, books, songs or simply using more Spanish during routines you're already doing.
Will bilingual babies talk later?
On average, no.
Dr. Byers-Heinlein explains that bilingual babies tend to reach their first-word milestones on a similar timetable to monolingual babies.
What can look different is where their vocabulary lives.
Imagine a monolingual toddler knows around 50 words.
A bilingual toddler exposed approximately equally to English and Spanish might know roughly 25 words in English and 25 in Spanish.
Looking only at the English vocabulary could make it appear that the bilingual child knows fewer words.
But you're only looking at half of their language system.
Their vocabulary should be considered across both languages.
And yes — if your child says both agua and water, those represent two words they know.
Does mixing Spanish and English confuse babies?
No.
Language mixing — sometimes called code-switching — is a completely normal part of bilingual communication.
Dr. Byers-Heinlein's research has even offered a window into what happens inside toddlers' brains when they hear a language switch.
When approximately 18-month-old bilingual toddlers heard a sentence switch languages, their processing temporarily slowed slightly and their pupils widened — a physiological sign that their brains noticed something unexpected.
But that's exactly what's interesting.
They noticed the switch.
Their brains weren't treating Spanish and English as one jumbled-up language.
They knew something had changed.
Additional research has shown that children navigate language mixing remarkably well. Even highly proficient bilingual adults show small processing effects when languages switch.
Language mixing isn't evidence of confusion.
In many ways, it's evidence that the bilingual brain knows it is managing two languages.
The best bilingual activity is the one you'll actually enjoy
So should you read more books?
Sing more songs?
Sit on the floor playing?
Take your child to the park and narrate everything you see?
Dr. Byers-Heinlein's answer is refreshingly simple:
Do what feels natural for you and your child.
One parent loves reading. Another loves crafts. Another talks constantly while walking around the neighborhood. Another sings.
What matters is creating rich, interactive language experiences.
As she explains in the episode, language learning is about connection and enjoyment.
And play is particularly powerful because children learn more readily when they're interested and engaged.
Their interests even show up in their earliest vocabulary: the child obsessed with trucks learns vehicle words; the child fascinated by people learns everyone's names; another child's first word might be blueberry.
Follow their attention.
Then add language. Toys designed around bilingual play — like El Elefante Quiere, which teaches action verbs in English and Spanish through song, or Cuéntale al Sol, a musical counting tool in both languages — make it easy to weave language into the moments your baby already loves.
The "recipe" for raising a bilingual child
Toward the end of our conversation, Dr. Byers-Heinlein distills decades of bilingual language research into something surprisingly straightforward:
Quantity + quality.
Children need enough exposure to each language.
And that exposure should happen through meaningful, interactive language in everyday life.
Not vocabulary drilled without context.
Not perfection.
Real conversation.
Real people.
Real life.
Talk while you're making breakfast. Sing the same song for the hundredth time. Name what your baby is reaching toward. Read together. Call their grandparents. Play on the floor. Speak the language you hope becomes part of their life.
Because babies are capable of learning two languages — or even more — from the very beginning.
And perhaps the most reassuring lesson from the science is this:
You don't need a perfect bilingual parenting system. You need opportunities for your child to hear and use their languages with people they love.
About Dr. Krista Byers-Heinlein
Dr. Krista Byers-Heinlein is a Professor of Psychology at Concordia University, where she holds the Concordia University Research Chair in Bilingualism and Open Science and directs the Concordia Infant Research Lab. Her research focuses on infants and children growing up in bilingual environments, including how young bilinguals distinguish their languages and learn words across two languages.
She has authored more than 90 peer-reviewed articles on early bilingualism and is the author of The Listening Bilingual: Speech Perception, Comprehension, and Bilingualism.
Watch the full conversation
How Bilingual Babies Learn Two Languages | From the Womb to First Words
Bilingüe ft. Krista Byers-Heinlein, PhD