March 18, 2010 -- Extreme obesity has reached ''alarming'' levels among children, according to a new study that looked at the weights and heights of more than 710,000 children aged 2 to 19.

''The prevalence of extreme obesity was much higher than we thought," says the study’s lead author, Corinna Koebnick, PhD, a research scientist at Kaiser Permanente in Southern California, a large prepaid health plan.

''Seven percent of boys and 5% of girls -- that is scary," she tells WebMD. That was the overall prevalence of extreme obesity she found. Before the study findings, Koebnick says, she would have expected perhaps 3% to 5%.

And the 7% and 5% figures are overall. For some ethnic groups, the prevalence of extreme obesity was much higher -- up to nearly 12%.