Developmental biologists long ago established that the heart is the first embryonic organ to develop, usually within 10 weeks of conception, delighting pregnant women who can hear their child’s heartbeat very early in a pregnancy.

But for reasons that are not fully understood, fetal heart development sometimes goes awry. That leads to congenital heart defects, which are among the most common birth defects in America, affecting as many as one in 100 babies. Most of these defects are minor, but some, including holes between the chambers of the heart, require invasive surgery after birth.

Scientists and cardiologists would love to be able to halt or fix those defects earlier, even in the womb, but hearts develop so soon after conception that “the damage is done before we even know it’s started,” says Dr Patrick Y Jay, an associate professor of pediatrics and genetics at the Washington University School of Medicine who oversaw the new study.

So recently, scientists have begun looking at factors that may lower the risk of defects, including a focus on mothers, who provide their growing fetus’s environment.

Scientists have known for some time that in animals and people, older mothers are at greater risk of delivering babies with a heart defect than younger mothers. But no one has known whether it was the age of the mother or of her eggs that most affected that risk.

So as the preliminary step in the new study, which was published last week in Nature, Dr Jay and his colleagues performed an intuitive but, surprisingly, first-of-its-kind experiment. They gathered female mice that had been bred to have a genetically high risk of delivering pups with holes between the chambers in their hearts. Half of the mice were young. The other half were old, by mouse reproductive standards, approaching menopause.

The scientists then simply transplanted young ovaries, containing young eggs, into the older mice and old ovaries into the young mice and impregnated all of the animals.

The age of the ovaries and eggs turned out to play effectively no role in the mothers’ risk of delivering pups with heart problems. Young mice had a low risk, even if their ovaries and eggs were old. Old mothers had a much higher incidence of pups with heart defects, even if their ovaries and eggs were youthful.

This finding encouraged the researchers, Dr Jay says, because if the age-related risk had resided in the eggs, it would have been unalterable. Eggs can’t change. Mothers, at least potentially, can.

He and his colleagues then set out to determine what might be affecting the older mothers’ risk of delivering affected pups. They suspected metabolism. Aging metabolisms become less efficient, as anyone with a midlife waistline expansion knows, and the scientists wondered whether the older mothers’ declining ability to metabolise fats and sugar could be affecting uterine environment and, in consequence, fetal heart development.

But when they fed high-fat kibble to pregnant older mice, their risk of bearing young with heart defects remained almost exactly the same as in older mice eating normal kibble. Diet did not matter.

So the scientists next considered exercise, which affects metabolism along with many other aspects of a mother’s body, Dr Jay says. The scientists placed running wheels in the cages of female mice genetically predisposed to bear pups with heart problems. Some of these animals were young, the equivalent of teenagers in mouse terms; others were approaching menopause. All were allowed to run at will for several weeks before becoming pregnant.

The other young and old mice remained sedentary to act as controls.

When the animals delivered their pups, the older mothers that had run had far fewer young with heart problems than did older sedentary mice. In fact, their risk was equivalent to that of young mice. About 10 percent of young pregnant mice had sick pups, whether they ran or not, and about 10 percent of older, running mothers did likewise. More than 20 percent of the sedentary mouse mothers delivered pups with defective hearts.

The scientists do not know how exercise altered that risk equation, although they suspect that metabolic changes within the uterus are involved. Exercise also is known to change the workings of some genes and prompt the release of substances from muscles and other cells, Dr Jay says. These substances can move into the mother’s bloodstream and eventually to the fetus, and might be beneficial to its developing heart.

At the moment, however, these possibilities are purely speculative. Many factors can contribute to heart defects, and because mice are not people, scientists do not know if the same changes would occur in pregnant women who exercise.

But, Dr Jay says, he would recommend that healthy pregnant women and those contemplating pregnancy try to become physically active, if they aren’t yet. “There are so many potential health benefits” for both mother and developing child, he says, and few known downsides.

Consult your doctor, of course, before starting to exercise. And don’t worry if you can’t or don’t wish to run. Exercise scientists generally consider wheel running by mice to be equivalent to a brisk walk for us.

—©2015The New York Times