Researchers also found that if a mother experienced something before pregnancy that made her fear something specific, her baby will quickly learn to fear it too - through her odour when she feels fear.
In the first direct observation of this kind of fear transmission, a team from University of Michigan Medical School and New York University studied mother rats who had learned to fear the smell of peppermint.
The study showed that mother rats "taught" this fear to their babies in their first days of life through their alarm odour released during distress.
In the study published in the Proceedings of the National Academy of Sciences, researchers said they pinpointed the specific area of the brain where this fear transmission takes root in the earliest days of life.
"During the early days of an infant rat's life, they are immune to learning information about environmental dangers. But if their mother is the source of threat information, we have shown they can learn from her and produce lasting memories," said Jacek Debiec, the U-M psychiatrist and neuroscientist who led the research.
"Our research demonstrates that infants can learn from maternal expression of fear, very early in life," he added.
"Before they can even make their own experiences, they basically acquire their mothers' experiences. Most importantly, these maternally-transmitted memories are long-lived, whereas other types of infant learning, if not repeated, rapidly perish," Debiec said.
Researchers taught female rats to fear the smell of peppermint by exposing them to mild electric shocks while they smelled the scent, before they were pregnant.
Then after they gave birth, the team exposed the mothers to just the minty smell, without the shocks, to provoke the fear response. They also used a comparison group of female rats that didn't fear peppermint.
They exposed the pups of both groups of mothers to the peppermint smell, under many different conditions with and without their mothers present.
Using special brain imaging, and studies of genetic activity in individual brain cells and cortisol in the blood, they zeroed in on a brain structure called the lateral amygdala as the key location for learning fears.
The team also showed that the newborns could learn their mothers' fears even when the mothers were not present. Just the piped-in scent of their mother reacting to the peppermint odour she feared was enough to make them fear the same thing.
When the researchers gave the baby rats a substance that blocked activity in the amygdala, they failed to learn the fear of peppermint smell from their mothers.
This suggests, Debiec said, that there may be ways to intervene to prevent children from learning irrational or harmful fear responses from their mothers, or reduce their impact.