Men’s overall mean girth grew by 0.8 inches, to about 39.8 inches, and women’s rose by 1.5 inches, to 37.8 inches. Abdominal obesity is defined as a waist size greater than 40.2 inches for men and 34.6 inches for women.

But the gain was much larger between the first survey and the last for certain demographic groups. Experts with the CDC examined seven two-year cycles of the National Health and Nutrition Examination Survey, or NHANES, an ongoing government study. The new figures were published in a research letter in JAMA.

Waistlines of white women aged 40 to 49 grew by 2.6 inches during the time period, those of Mexican American men aged 20 to 29 expanded by 3.4 inches and the belt sizes of black women aged 30 to 39 climbed by 4.6 inches.

The new numbers are disturbing, said Dr. Anne McTiernan, an epidemiologist with Fred Hutchinson Cancer Research Center’s Public Health Sciences Division and an expert on weight control and physical activity.

“This is significant because it suggests that the trend to stabilizing obesity rates might not be true,” she said.

More than a third of adults in the U.S. – nearly 35 percent – are classified as obese based on body-mass index, a measure of weight corrected for height, according to previous CDC reports. But researchers have been saying since 2010 that it appeared that the American obesity epidemic had actually leveled off. The new data suggest that the BMI reports might be missing something, McTiernan said.

“Waist size is one more measure of obesity and for some health conditions, it might be as important as weight,” she said. “Individuals with large waist measurements have increased risks for heart disease, diabetes and several kinds of cancer, such as breast, endometrium and advanced prostate cancer.”

It’s not exactly clear what might be behind the mixed reports of stable BMI and expanded waistlines, Ford said. Some researchers have suggested that sleep deprivation, endocrine-disrupting chemicals and certain medications may have contributed to the growing abdominal girth.

In rare cases, a person indeed could have a high BMI, but a slim waist. As an example, Ford cited champion body builder and former California Gov. Arnold Schwarzenegger, who was 6-foot-2 and weighed 240 pounds in his prime. That gave him a BMI of nearly 31, technically obese -- but his waist was just 34 inches, far below the bar for abdominal obesity, Ford noted.

Most ordinary people, however, have BMIs and waist sizes that are more closely aligned, Ford noted.

Clinical guidelines from the National Heart, Lung and Blood Institute have recommended since 1998 that measuring waist circumference be part of doctors’ assessments of their overweight patients. The new data suggest that a using a measuring tape more often might be not only warranted, but crucial.

“At a time when the prevalence of obesity may have reached a plateau, the waistlines of U.S. adults continue to expand,” they wrote. “Our results support the routine measurement of waist circumference in clinical care consistent with current recommendations as a key step in initiating the prevention, control and management of obesity among patients.”

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JoNel Aleccia is a staff writer at Fred Hutchinson Cancer Research Center. From 2008 to 2014, she was a national health reporter for NBC News and msnbc.com. Prior to that she was a reporter, editor and columnist for more than two decades at newspapers in the Northwest. Reach her at jaleccia@fredhutch.org.