Why Bilingual Mothers Sing Differently to Their Babies — And What It Means

A baby in the womb begins to pay attention to her mother’s voice at some point during the third trimester. Not only does she hear the sound of a passing bus or the dull thud of a slammed door, but she also sorts the sound waves that pass through skin and amniotic fluid into something that starts to feel meaningful. We already knew this. Up until now, we had no idea what would happen if that voice continued to switch between two languages.

This question has been the focus of a team of researchers in Barcelona, and their results, which were recently published in Frontiers in Human Neuroscience, offer something truly unexpected. It turns out that bilingual babies perceive the world in a different way than monolingual babies. Not better. Not worse. Simply put, the difference manifests itself within hours of birth.

Catalonia is a logical location for this kind of study. In daily life, about 42% of the population speaks both Spanish and Catalan, frequently switching between the two without realizing it. One to three-day-old infants from Sant Joan de Déu Children’s Hospital were gathered by the team, who then applied tiny electrodes to their tiny foreheads. Most of the time, the infants slept. They fussed occasionally. The frequency-following response, a type of electrical fingerprint of how accurately the brain mimics a sound it hears, was measured by the machines.

The sound itself was a deliberate little construction, consisting of a 250-millisecond clip of vowels with a rising pitch at the end and a shift from /o/ to /a/. Spanish and Catalan both have these vowels, which was important. Additionally, low-frequency sounds are more effective at passing through the womb than high-frequency ones, which was more significant.

Observing the squiggles on their screens, the researchers discovered a sort of trade-off. The brain response to the sound was sharper and cleaner in babies whose mothers had only spoken one language during pregnancy. Their hearing systems were locked in. Babies whose mothers spoke two languages—Spanish and Catalan, or occasionally Spanish and Arabic, English, Romanian, or Portuguese—exhibited a more expansive but less precise reaction. They were picking up less of any one thing while listening to more.

It’s difficult not to interpret that as almost philosophical. A baby who speaks only one language is born perfectly tuned. When a bilingual baby is born, they are more receptive, inquisitive, and possibly less certain. If there were investors in human potential, they might not agree on which is a better place to start.

One of the main authors, Dr. Natàlia Gorina-Careta, was cautious in her wording. According to her, bilingual babies seem “more sensitive to a wider range of acoustic variation,” whereas monolingual babies appear more “selectively tuned.” Dr. Jordi Costa Faidella, her colleague, was even more cautious. Gently, he cautioned that this should not be used as a guide for parenting. Long after birth, the brain continues to learn. It is only the initial sketch, whatever shape it takes in the womb.

Nevertheless, the notion that a fetus is already performing this kind of work while floating in the dark has a subtle, poignant quality. In some unconscious way, it is already choosing between spreading itself thin and specializing. Researchers at Princeton discovered a similar phenomenon in older infants: mothers speaking nine different languages all changed the timbre of their voices when speaking to their babies, as if an old instinct took over. The body is aware. It remembers, at least.

It’s still unclear if that initial difference will matter in the long run. The majority of us learn our mother tongues regardless of how the wiring began, and the sensitive period for language extends well beyond birth. However, it’s important to keep in mind that someone is listening the next time you see a pregnant woman conversing with her unconscious belly in two languages and to learn how to listen in a specific way.