Infants born prematurely are more than twice as likely to have difficulty hearing and processing words than those carried to full-term, likely because brain regions that process sounds aren’t sufficiently developed at the time of delivery. Now, an unusual study with 40 preemies suggests that recreating a womblike environment with recordings of a mother's heartbeat and voice could potentially correct these deficits.

"This is the kind of study where you think ‘Yes, I can believe these results,’ " because they fit well with what scientists know about fetal brain development, says cognitive scientist Karin Stromswold of Rutgers University, New Brunswick, in New Jersey.

A fetus starts to hear at about 24 weeks of gestation, as neurons migrate to—and form connections in—the auditory cortex, a brain region that processes sound, Stromswold explains. Once the auditory cortex starts to function, a fetus normally hears mostly low-frequency sounds—its mother’s heartbeat, for example, and the melody and rhythm of her voice. Higher frequency tones made outside of the mother's body, such as consonants, are largely drowned out. Researchers believe that this introduction to the melody and rhythm of speech, prior to hearing individual words, may be a key part of early language acquisition that gets disrupted when a baby is born too soon.

In addition to being bombarded with the bright lights, chemical smells, and shrill sounds of a hospital’s intensive care unit, preemies are largely deprived of the sensations they'd get in the womb, such as their mother's heartbeat and voice, says Amir Lahav, a neuroscientist at Harvard Medical School in Boston. Although mothers are sometimes allowed to hold premature newborns for short periods of time, the infants are often considered too fragile to leave their temperature- and humidity-controlled incubators, he says. Preemies often have their eyes covered to block out light, and previous studies have shown that reducing overall levels of high-frequency noise in a neonatal intensive care unit—by lowering the number of incubators in a unit, for example, or giving preemies earplugs—can improve premature babies' outcomes. Few studies have actively simulated a womblike environment, however, he says.

To test whether the sounds a fetus would hear in utero can have a positive effect on preemies, Lahav and his colleagues asked the parents of 40 such babies at Brigham and Women's Hospital in Boston for their participation in a monthlong experiment. The researchers asked mothers of half the infants to sing and read "Twinkle, Twinkle, Little Star" and Goodnight Moon in a recording studio and record their heartbeats through a stethoscope connected to a microphone. The scientists then removed the higher frequency tones from the recordings and piped the remaining sound for 45-minute sessions totaling 3 hours per day into 21 infants' incubators, while the other infants received standard care. After 30 days, they compared ultrasound images of the brains of both groups.

Babies exposed to their mothers’ voices had significantly thicker auditory cortices than those in the control group, the researchers report online today in the Proceedings of the National Academy of Sciences. Although the enhanced growth doesn't prove anything about how the infants will do later on, "we know from other studies that bigger is better" when it comes to brain development in this region, Lahav says. Next, he says, the team plans to track the infants' through school age to see if they develop hearing or speech problems as they grow up.

The fact that the researchers saw differences in brain structure after just 30 days fits well with existing theories about premature infants' brain development, Stromswold says. The study has several weaknesses, however, she notes. One is that there were five more females in the treatment group than in the control group. Female preemies tend to do better than males do, especially where language is concerned, so the imbalance could have skewed the group's analysis to favor a positive result. Another is that the group did not compare "before" and "after" ultrasounds to see how each individual infant's brain changed over time, she says. Instead, "all they’re saying is that after 30 days of exposure, we find a difference between groups." The preemies did receive routine ultrasounds immediately after birth to rule out brain injuries, Lahav says, but the images were too coarse to use for comparison.

Lahav agrees that larger and more detailed studies are needed to confirm his team’s results, and he cautions that such interventions may never be enough to counteract the many health problems preemies face. Still, he says, the study suggests that even 3 hours per day of exposure to womblike sounds "may be enough to set the brain on the right developmental track."