When does memory begin? We can’t consciously call up images from our infancy, but we surely learn important, lasting associations at very early ages. New work suggests this type of memory begins even in the womb.

In a study published in July in Child Development, researchers from the Netherlands reported short-term memory in 30- to 38-week-old fetuses. First they put a vibrating, honking device on the abdomens of 93 preg­nant women. The fetuses quickly “habituated”—that is, they figured out that the noise was not dangerous. When they heard it again 10 minutes later, they did not squirm and their heart rates did not escalate. “It’s like getting used to a New York train sta­tion,” says lead author J. G. Nijhuis, a professor of obstetrics at Maastricht University in the Netherlands. “It is a learning capability to distinguish safe from unsafe stimuli. It is a primitive form of memory.”

The 34-week-old fetuses even recalled the sound four weeks later. “What this study clearly says is at least beginning at 30 weeks and pos­sibly before that, the fetal brain is starting to lay down short-term memo­ries and might even be laying down some long-term memories,” says Rahil Briggs, director of Healthy Steps at Montefiore Medical Center and assis­tant professor of pediatrics at Albert Einstein College of Medicine. “This is a sensitive period of development.”

Fetuses habituate in other ways, too. Substance-abusing moms give birth to drug-addicted babies. A study found that the babies of mothers who watch a popular Spanish-language soap opera while pregnant calm down when they hear the show’s theme music. And anecdotally, some dads who read to fetuses in the womb think their babies are born recognizing their voices, says pediatrician Tanya Remer Altmann, a spokesperson for the American Academy of Pediatrics.

The bottom line: be conscientious around the baby-to-be. “The environ­ment in utero, and extra utero, is very important,” says pediatrician Dimitri Christakis, director of the Center for Child Health, Behavior and Develop­ment at Seattle Children’s Hospital. After all, the brain triples in size in the first two years of life. And perhaps even younger fetuses develop memories—researchers will investigate that pos­sibility next.

Note: This story was originally printed with the title "Recall in Utero"