The reason: She may have her first period later, delaying her exposure to peak levels of estrogen, says Leslie Bernstein, Ph.D., a cancer researcher at the University of Southern California, in Los Angeles. She studied girls ages 9 to 13 over four years. Those who were moderately active for at least four hours a week -- not just sports, but informal time running around the playground -- began menstruating latest.
While delaying puberty by a year lowers lifetime breast-cancer risk by only a modest 6 to 8 percent, there are other benefits to keeping little girls moving. "Physical activity also reduces the risk of childhood obesity and, in adulthood, heart disease, depression, and Type 2 diabetes, as well as other cancers," says Dr. Bernstein.