Newborn babies respond differently to their mother tongue as compared to foreign languages thanks to all the listening they did while in the womb, a joint study by American and Swedish researchers suggests.

The results of the study, which will be published in a forthcoming issue of the peer-reviewed pediatrics journal Acta Paediatrica, suggest that fetuses listen to the particular speech sounds their mothers make during the later stages of her pregnancy, and can show signs of what they learned shortly after birth.  

"This study moves the measurable result of experience with speech sounds from six months of age to before birth," lead author Christine Moon, a psychology professor at Pacific Lutheran University in Tacoma, Wash., said in a statement.

Moon described the study as the first to show that fetuses learn about these sounds prenatally.

About 30 weeks into gestational age, fetuses develop sensory and brain mechanisms for hearing, the authors note, adding that they are listening to "ambient language" or speech — most clearly that of their mother — 10 weeks before they are born.

Patricia Kuhl, co-director of the University of Washington’s Institute for Learning & Brain Sciences, adds that vowel sounds uttered by the expectant mother are particularly stimulating.

"The vowel sounds in her speech are the loudest units and the fetus locks onto them," said Kuhl who, along with Swedish professor Hugo Lagercrantz, co-authored the study.

Early curiosity

The researchers gave wired pacifiers to 80 newborns who were between seven to 75 hours old: a group evenly split between boys and girls in both the U.S. and Sweden, and who were born to monolingual mothers.

They noted that the babies had different reactions to familiar and foreign vowel sounds, as assessed by how vigorously they sucked on their pacifiers while listening computer-generated variants of either the English /i/ or Swedish /y/. 

The babies were able to control how many times they heard a vowel sound, and for how long, by sucking on their pacifier.

The researchers found that newborns in both countries sucked longer when exposed to foreign vowel sounds not present in their mother tongue.

"Each suck will produce a vowel until the infant pauses, and then the new suck will produce the next vowel sound," said Kuhl, who added that the infants seemed more interested in the foreign sounds. "At birth, they are apparently ready for something novel."

"Babies are not machines and many things besides the sounds affect sucking, so it's not that simple, but it works well enough that when you have 80 babies, and you count average sucks," Moon said in an email to CBC News.

In the study's manuscript, the authors note that there were no statistically significant differences between babies of different genders and birth weights.

This study builds on previous research that suggests babies begin learning about language immediately after birth, and start to distinguish between languages within their first few months, by suggesting that babies are born with some of that familiarity.

"This study points to a new direction in studying early experience to language, and more studies with different speech sounds and language communities will clarify what is learned prenatally about language sounds," said Moon.