Reporter: Run way models have it. Ultra thin strend certificates like kate moss have it. What is it? The latest obsession for a shocking number of teenage girls. What is a thigh gap? A thigh gap is... See More
Reporter: Run way models have it. Ultra thin strend certificates like kate moss have it. What is it? The latest obsession for a shocking number of teenage girls. What is a thigh gap? A thigh gap is you're standing straight up, your feet and knees are together, there's a gap in between your thooigs. Girls don't want their thighs to touch. Reporter: Another way for teen girls to scrutinize themselves. The movie "mean girls" explored it. Hi hips are huge. Oh, please, I hate my calves. Reporter: "Good morning america" sat down with four high school juniors to try to get signed the mind of teenage girls. What does it symbolize if you have a thigh gap? I guess it's the ideal body shape. Reporter: What kind of fashion pieces though it off? Tight jeans or leggings. Reporter: It follows teen home. Tumblr, facebook, and twitter are flooded with images of thigh gap. Confidence-crushing messages disguised as inspiration for staying thin. This thigh gap will get bigger. The popular wruns skinny legs. It's a site that makes you feel like you have to be part of that group. You have to be skinny. Have wavy hair. I see the pictures and think, wow, they look so good. Then I realize how unhealthy it is. Reporter: Tumble says it discourages blogs like this. They're setting themselves up for an unattainable goal. That's right. Reporter: And the motivation for extreme body images? Not what some might expect. It's a popularity thing? The popular girls are pretty, skinny, long hair opinion. Reporter: You to think boys look at a girl's thigh gap? No. They don't know what it is. Reporter: They don't know what it is, right? The boys don't know what it is. So why do girls care? It's a girl thing. Reporter: Is it a girl status thing? I would say so. Reporter: Status thing that leaves too many girls feeling like they don't measure up. For "good morning america," juju chang, abc news, new york.
This transcript has been automatically generated and may not be 100% accurate.