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Phys Ed Gretchen Reynolds on the science of fitness.

If a woman is physically active during pregnancy, she may boost the development of her unborn child’s brain, according to a heart-tugging new study of expectant mothers and their newborns. The findings bolster a growing scientific consensus that the benefits of exercise can begin to accumulate even before someone is born.

It has long been suspected that a mother-to-be’s activity — or lack of it — affects her unborn offspring, which is not surprising, given how their physiologies intertwine. Past studies have shown, for example, that a baby’s heart rate typically rises in unison with his or her exercising mother’s, as if the child were also working out. As a result, scientists believe, babies born to active mothers tend to have more robust cardiovascular systems from an early age than babies born to mothers who are more sedentary.

Whether gestational exercise similarly shapes an unborn child’s developing brain has been harder to quantify, although recent studies have been suggestive. An experiment presented this month at the Society for Neuroscience’s annual meeting in San Diego, for instance, reported that pregnant rats allowed to run on wheels throughout their pregnancies birthed pups that performed more dexterously in early childhood on a tricky memory test — having to identify unfamiliar objects in a familiar environment — than pups born to sedentary moms. These clever rats retained their cognitive advantage into adulthood (meaning, for rats, weeks later).

But this and similar experiments have involved animals, rather than people. Many of these studies also began comparing the creatures’ cognitive abilities when they were old enough to move about and respond to their world, by which time they potentially might have been shaped as much by their environment as by their time in the womb.

So to minimize these concerns, researchers at the University of Montreal in Canada recently recruited a group of local women who were in their first trimester of pregnancy. At that point, the women were almost identical in terms of lifestyle. All were healthy, young adults. None were athletes. Few had exercised regularly in the past, and none had exercised more than a day or two per week in the past year.

Then the women were randomized either to begin an exercise program, commencing in their second trimester, or to remain sedentary. The women in the exercise group were asked to work out for at least 20 minutes, three times a week, at a moderate intensity, equivalent to about a six or so on a scale of exertion from one to 10. Most of the women walked or jogged.

Every month, for the remainder of each woman’s pregnancy, she would visit the university’s exercise lab, so researchers could monitor her fitness. All of the volunteers, including those in the nonexercise group, also maintained daily activity logs.

After about six months and following the dictates of nature, the women gave birth. All, thankfully, had healthy boys or girls — which the scientists gently requested that the mothers almost immediately bring in for testing.

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Within 12 days of birth, in fact, each of the newborns accompanied his or her mother to the lab. There, each baby was fitted with an adorable little cap containing electrodes that monitor electrical activity in the brain, settled in his or her mother’s lap, and soothed to sleep. Researchers then started a sound loop featuring a variety of low, soft sounds that recurred frequently, interspersed occasionally with more jarring, unfamiliar noises, while the baby’s brain activity was recorded.

“We know that baby’s brains respond to these kinds of sounds with a spike” in certain types of brain activity, said Elise Labonte-LeMoyne, a Ph.D. candidate at the University of Montreal, who led the study and also presented her findings at the Society for Neuroscience annual meeting. This spike is most pronounced in immature brains, she continued, and diminishes as a newborn’s brain develops and begins processing information more efficiently. “It usually disappears altogether by the time a baby is 4 months old,” she said,

In this case, the relevant brainwave activity soared in response to the novel sounds among the children born to mothers who had remained sedentary during pregnancy. But it was noticeably blunted in the babies whose mothers had exercised. In essence, “their brains were more mature,” Ms. Labonte-LeMoyne said.

How gestational exercise can remodel an unborn child’s brain is not clear, Ms. Labonte-LeMoyne admits, since, unlike circulatory systems, a mother’s brain is not hardwired directly to that of her child. “But we suspect that when mom exercises, she generates a variety of chemicals,” including many related to brain health, which can move into her bloodstream and eventually mingle with the blood of her baby.

But that possibility is only theoretical for now. It is also unclear whether the precocious brain development seen in newborns with active mothers will linger into their later lives. Ms. Labonte-LeMoyne and her colleagues plan to retest the children on various cognitive tests once they are a year old.

But for now, the lesson is clear. “If a woman can be physically active during her pregnancy, she may give her unborn child an advantage, in terms of brain development,” Ms. Labonte-LeMoyne said. And the commitment required can be slight. “We were surprised,” she said, “by how much of an effect we saw” from barely an hour of exercise per week.