A new body trend is apparently becoming an obsession among teenage girls.

It's the thigh gap - a clear space, or gap, that can be seen between the thighs when a girl is standing with her knees together. Some runway models have it, and teen girls want it.

"Good Morning America" sat down with four high school juniors from a New Jersey Chapter of Students Against Destructive Decisions to discuss this latest trend.

The four girls told ABC News' Juju Chang that they all had friends that were intent on achieving the thigh gap.

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Emily Rozansky told Chang that, for many teens, the thigh gap symbolized "the ideal body shape."

Social media sites such as Tumblr, Facebook and Twitter have devotees who flood the zone with images of thigh gaps, bony collarbones and confidence-crushing messages disguised as "inspiration" for staying thin.

Some of the most popular pictures showcase very thin girls with protruding hip bones and a thigh gap.

The teens told Chang that the sites make them feel they have to conform.

Angela DePalma, 16, said: "I see those pictures on Tumblr and stuff and I think that wow, like, they look so good. And then I realize how unhealthy that is."

Tumblr says it discourages blogs that actively promote or glorify self-harm.

According to teen psychologist Barbara Greenberg, statistics show that 80 percent of girls dislike their bodies by the time they are 17 years old. That, combined with a tendency to overshare, makes teen girls vulnerable to even the most subtle messages.

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A quick online search brings up page after page of thigh gap inspiration photos and supporters. Experts say the images alone can lead to self-destructive behavior, especially since the thigh gap is, for most girls, an unrealistic standard of beauty.

Greenberg said teens who were pursuing a thigh gap were "setting themselves up for not only an unattainable goal but for an unsustainable goal.

"Even if they reach it, it's going to be very hard for them to maintain it," she said.

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Surprisingly, some girls' motivation to have a thigh gap isn't to make themselves more desirable to boys. The New Jersey students told Chang that some boys don't even notice it.

It's strictly a girl thing, affecting popularity and status, they said.