It’s official: While still in utero, babies are preparing for the moment when they can sass back. A new study found fetuses at eight months have differing physical responses to different languages — evidence that language development starts in the womb.

Previous research had determined that language development in children starts within a few days of birth, and while a single study suggested fetuses could discriminate between different types of language based on rhythmic patterns, that study used different speakers for each language, calling into question whether the fetuses were responding to the difference in language or voice.

“This early discrimination led us to wonder when children’s sensitivity to the rhythmic properties of language emerges, including whether it may in fact emerge before birth,” said Utako Minai, associate professor of linguistics at the University of Kansas and the team leader on the study. “Fetuses can hear things, including speech, in the womb. “It’s muffled, like the adults talking in a Peanuts cartoon, but the rhythm of the language should be preserved and available for the fetus to hear, even though the speech is muffled.”

Two dozen American women, averaging roughly eight months pregnant, were examined using ultrasensitive, yet non-invasive magnetocardiogram (MCG) technology, which detects tiny magnetic fields that surround electrical currents from the mother’s and fetus’s movements, including heartbeats and breathing.

“Obviously, the heart doesn’t hear, so if the baby responds to the language change by altering heart rate, the response would be directed by the brain,” said team member Kathleen Gustafson, a research neurologist at the university medical center’s Hoglund Brain Imaging Center.

Which is exactly what the study found when Minai had a bilingual speaker make two recordings, one each in English and Japanese, to be played in succession to the fetus. (English and Japanese are considered to be rhythmically distinctive; English speech has a dynamic rhythmic structure resembling Morse code signals, while Japanese has a more regular-paced rhythm.) Fetal heart rates changed when fetuses heard the unfamiliar, rhythmically distinct language (Japanese) after having heard a passage of English speech, while their heart rates did not change when they were presented with a second passage of English instead of a passage in Japanese.

“Fetuses are tuning their ears to the language they are going to acquire even before they are born, based on the speech signals available to them in utero,” Minai said. “Pre-natal sensitivity to the rhythmic properties of language may provide children with one of the very first building blocks in acquiring language.”

Proof that the little one is listening in before he pops out. And you thought you still had a month before you had to stop swearing.

Robert Fiorentino, Allard Jongman and Joan Sereno also contributed to the study, which was published in the journal NeuroReport.