"The intrusion and presence of social media in our lives really does make it very difficult," said Nancy Albus, chief executive officer of Castlewood Treatment Center, a suburban St. Louis facility that focuses on eating disorders. "The important distinction about thigh gap is it gives you an actual visual to achieve, this visual comparison of how your body does or doesn't stack up."

Vonda Wright, a Pittsburgh-based orthopedic surgeon and fitness expert, said the spacing between a person's legs is based mostly on genetics. And even extraordinarily thin people may not have a body type that can achieve a gap. You have to be both skinny and wide-hipped, she said.

Besides, Wright said, it isn't a goal worth chasing. Most fit people won't have a thigh gap because their thighs are muscular enough that they touch, she said.

"Skinny does not mean fit or muscular," said Wright, who works with Division I athletes. "I cannot think of one athlete I deal with" who has a thigh gap.