It's the newest weight-loss craze with teens -- achieving the “thigh gap.”
"It's if you were to stand up -- it's the gap between your thighs -- and the goal and the pursuit of thigh gap is to ensure that your thighs do not touch,” said Nancy Albus, CEO of Castlewood Treatment Centers in St. Louis, Mo.
Teens are posting pictures of their legs on sites like Tumblr.
There's even a Twitter account that someone made for super models' thigh gap. It has almost 3,000 followers.
While some are born with this kind of bone structure, adolescent medicine specialist Dr. Sharona Taylor worries that other teens will go too far to try to achieve it.
"When someone's lost too much weight, they can affect their overall growth and development," she said. "From brain functioning to how their heart functions, they may end up with low blood pressure, low heart rates, low body temperatures."
"They may lose their periods altogether because nutritionally their bodies cannot support basic functioning."
Dr. Taylor urges parents to talk to their teens about body obsession and normal development.
"A lot of times we prepare our girls for puberty -- talking about bras and periods and not talking about body composition changes in overall body changes as well," she said.
Experts say watch for changes in your children. Do they have a sudden interest in diet and exercise? Are they doing extreme or fad dieting?
If you're concerned, seek medical help.
Medically speaking, a thigh gap has more to do with a wide pelvis than with weight.