It's no surprise that many women say their weight problems began during their teenage years. In the past, hormones, puberty, and bad eating habits took most of the blame. But a new study published in the journal The Lancet questions this old wisdom. It found that a sharp decrease in girls' activity levels during the teen years — rather than what they're eating or how their bodies are changing — is what's to blame! Researchers discovered that activity levels dropped sharply once girls became teenagers — what followed was a 10- to 15-pound weight gain.

More research is needed to determine why teenage girls suddenly slow down as they move from "childhood" activities to adult ones, like shopping or talking on the phone. However, many factors might be to blame, from peer pressure to a lack of opportunity. So how can you help your teen avoid the inactive trend? Get your adolescent involved in an activity or sport that keeps her moving. From volleyball and gymnastics to dance and swimming — there are many fun and active ways for teens to pass their leisure time besides lounging on the couch!