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"I thought on prom night I'd be alone, in front of the TV, eating ice cream," said Cox.
Nine months ago, the high school senior tipped the scales at 291 pounds.
"Add one of my friends onto me, and that was me," said Cox.
Jen tried dieting, but family dinners at fast-food joints kept her from losing weight. Then her parents had gastric bypass surgery.
"I was like, 'This might be something I want to do,'" said Cox.
After seeing her parents lose weight, Jen decided to get the surgery. Most people lose 50 to 80 percent of their excess weight in two years. The surgery makes the stomach the size of a thumb.
"Take a medicine cup, and that's what you can eat," said Cox.
The obesity rates for children and teens have tripled in the past 20 years. The numbers show they're increasingly turning to surgery. About 200 teens had bariatric surgery in 2000. Three years later, the number jumped to 800.
"We need to track patients long-term to see how they do throughout life," said Dr. James Lau from the University Medical Center of Southern Nevada.
Complications from gastric bypass include infection, painful swelling and heart problems. Dr. Lau says the surgery should not be performed on teens until their bones are done growing.
She lost 97 pounds in nine months. Those daily walks mean she won't have to miss out on teenage traditions.
"I just bought my prom dress like two weeks ago," said Cox.
Not just any teenager qualifies for bariatric surgery. They have to be emotionally and physically mature, and have medical problems associated with obesity. And teens should participate in an extensive, medically-supervised weight loss program before even being considered for surgery.