Exercising during pregnancy might have unanticipated benefits – at least in mice, a new study suggests. Pups born to active moms developed bigger brains a few weeks after birth.

Tantalisingly, the growth was confined to a very specific part of the brain linked with intelligence. Compared with the offspring of inactive mothers (those denied an exercise wheel) pups born to active mothers typically developed 40% more cells in the hippocampus, the area of the brain vital for learning and memory.

But the extra cells came after birth. The pups from exercising mums actually grew fewer hippocampus cells while in the womb, and had a lower body weight when born. But more than a month after birth, the pups from active mums had more than made up for it, putting on a spurt which gave them the extra 40% of hippocampal cells overall.

The researchers were not able to tell whether the pups of active mums were more intelligent in this experiment because the mice were sacrificed in order to examine their brains.

“We’d have to replicate the experiment, and that would take two years,” says Gerd Kempermann of the Max Delbrück Center for Molecular Medicine in Berlin, Germany, and head of the team.

Kempermann says previous studies in rodents have demonstrated that when adults exercise, they develop extra cells in the hippocampus. One reason might be that active rodents are both fitter generally, but also more likely to encounter novel situations as they rove which demand extra intelligence and memory. “Now, we’ve established that it seems to be transmissible to offspring,” he says.

He warns against extrapolating the results to people, but suggests "we should have another look at human sport and exercise during pregnancy”.