A handful of preliminary studies in recent years has raised hopes the epidemic of U.S. child obesity has stabilized or reversed. But new research finds continued growth in our kids’ girth, suggesting that self-congratulation would be premature.

Among children from infancy through age 18, rates of obesity have increased steadily from 1999 to 2014, and the numbers of children with the severest forms of obesity have risen most dramatically, a new study finds.

About a third – 33.4% of American children in 2014 – were overweight, and just over half of those kids (17.4% of all children) weighed in as obese (which is defined as being at or above the 95th percentile of height and weight on age- and gender-specific growth charts).

While most obese children are classified as having Class I obesity, about a third of obese kids (or 6.2% of all U.S. children) are thought to have severe obesity (either Class II obesity – at or above 120% of the 95th percentile on the growth charts – or Class III obesity, which is diagnosed when a child is at or above 140% of the 95th percentile for his or her age).

Recognizing weight problems or obesity in a child is complex, and parents are notoriously bad judges of whether their children’s weight is worrisome. Here’s an online calculator that will tell you definitively where your child or adolescent lies on the spectrum.

Rates of obesity among all American children have risen from 14.6% in 1999-2000 to 17.4% in 2013-14. The severely obese were a small slice of the population in 1999. But the percentage of children categorized with Class II and Class III obesity has risen steeply over the same period: from 4% to 6.3% for Class II-or-above obesity and from 0.9% to 2.4% for Class III obesity.

The new study, published Monday in the journal Obesity, dims optimism prompted by research published two years ago, when a similar population-wide study found a large drop in obesity rates among toddlers, ages 2 to 5. The decline was hailed as the possible leading edge of a reversal in U.S. child obesity – a positive response to public health initiatives aimed at driving down the nation’s girth.

The authors of the current study, researchers at Duke University, University of North Carolina and Wake-Forest University, dashed even that small glimmer of hope. The reported decline, they wrote, “is not evident in our results, for girls or boys.”

Amid widespread efforts to improve kids’ diets and boost their physical activity levels, other studies have documented stabilization or reversal of obesity rates in smaller segments of U.S. children, including in California, New York, Philadelphia and in some counties of Mississippi.

Those pockets of progress may still exist. But the current study suggests they are being washed out by continuing, surging growth in child obesity rates.

All told, 4.5 million U.S. children and adolescents are now considered severely obese, and they’ll need “novel and intensive efforts for long-term obesity improvement,” the researchers wrote.

These are children who are overwhelmingly likely to remain very heavy into adulthood. And since even youngsters show early signs of cardiovascular disease and metabolic disorders when they carry so much extra weight, they are likely to become adults with a heavy burden of chronic diseases – and very likely shortened lifespans.

Duke University’s Asheley Cockrell Skinner, a childhood obesity expert and lead author of the study, called the hike in severe obesity the study’s “most disheartening” finding. While slowing a child’s weight gain and reversing his or her obesity is a challenge, Skinner said, children and their parents should not feel alone in the fight.

“This is really a population health problem that will require changes across the board – food policy, access to healthcare, school curriculums that include physical education, community and local resources in parks and sidewalks,” Skinner said. “A lot of things put together can work.”

Earlier this month, Harvard University child obesity specialist Dr. David Ludwig warned that a 2015 surge in deaths across the age span from obesity-related diseases might signal “a looming social and economic catastrophe that demands a comprehensive national strategy.” Unless public officials and Americans put health concerns before commercial ones, Ludwig wrote, decades of vaulting progress in extending U.S. lifespans will be reversed by the epidemic of obesity.

Among adolescents in the current study, 1 in 10 were defined as having Class II-or-above obesity in 2013-14, and half of those were defined as suffering Class III obesity – the most extreme. Although boys and girls were roughly equally likely to be obese, girls were more likely than boys to be severely obese. And the proportion of girls in those categories grew faster than that among boys.

The new study confirms that obesity rates are either growing fast or stubbornly high within ethnic minority communities.

Among African American girls, very high rates of obesity – as high as 24.4% in 2005-06, have dropped slightly. Among black girls, 20.9% were obese in 2013-14 (of whom almost 60% had Class II or Class III obesity). Latino girls have overtaken African American girls in obesity rates, going from 15.4% obese in 1999 to 22.1% in 2013-14 (about half of whom were severely obese).

Among non-Latino white girls, 14.8% were obese in 2014 – up from 12% in 1999-2000.

White and black boys were roughly equally likely to be obese (16.6% vs. 16.8%), although obese African American boys in 2014 were far more likely than obese white boys to have severe obesity. Fully 21.2% of Latino boys were obese in 2013-14, and close to 1 in 4 such boys was considered severely obese.