Keep your eye on this statistic, the staple context for any story about America's fat youth. It refers to the fact that roughly one in six children and teenagers is classified as "obese," and another one in six is considered "overweight."

According to the government, an obese child is one whose body mass index—that's weight divided by height squared—is at or above the 95^th percentile for age and gender. An overweight child falls between the 85^th and 94^th percentiles. So, in theory, the nation's overweight and obese children would represent the fattest 15 percent of the population. In fact, they account for 32 percent. Why is that?

The percentiles used to describe the size of children are based on a hoary data set that's 40 years old. When parents take a newborn to the pediatrician, they're given a set of rankings for their baby's length, weight, BMI, head circumference, and so on. These numbers come from government-issued growth charts (PDF), built from population surveys conducted in the 1960s and 1970s. In other words, the doctor isn't telling you how your baby compares to other babies today; she's saying how your baby stacks up against babies from the Nixon era.

Since infants (and adults) have gotten bigger over the past few decades, modern size scores tend to be shifted to the right. So a kid who shows up in the 50^th percentile on the growth charts wouldn't be in the middle of the distribution; he'd be small for his age. The fact that we're ranking children with old data gives rise to a Lake Wobegon effect: Most babies and kids today are in the upper percentiles for body size—they're "above average."

This practice might seem a bit misleading, but it comes in handy when you're talking about public health. By using a benchmark standard for body size, we're able to keep tabs on how the population changes over time. If we updated the charts with new data every year, then the child obesity rate would be stuck at 5 percent, no matter how fat our kids became on average.

Still, it's worth remembering what the numbers really mean: When we say that one in three children is now overweight or obese, we're not basing that figure on some objective measure of health risk. We're saying that 32 percent of today's kids match up in size with the biggest 15 percent from the 1960s and 1970s.

Pretty soon, parents are going to be hearing a lot more about these BMI percentiles. In concert with the first lady's campaign, the American Academy of Pediatrics has called for regular monitoring of body mass index among kids. Ten states are now doing some limited BMI surveillance in public schools, and three more—Arkansas, Massachusetts and Pennsylvania—have passed laws requiring annual BMI screenings for all students. So what does it mean if your child shows up in the 85^th percentile?

It's hard to say. We do know that the very fattest children tend to become obese adults, and obese adults face a significant risk for diabetes, heart disease, and early death. A study published last week in the journal Pediatrics suggests that the development of obesity-related diseases may begin as early as age 3. That is to say, toddlers in the 95^th percentile for BMI show some of the blood-level markers associated with heart disease in adults. But it's unknown whether even these childhood markers—or childhood obesity in general—can predict specific health problems down the road.

At the same time, the relationship between childhood BMI and health can be somewhat misleading. A paper published last September looked at how a kid's body mass index relates to his percent body fat. We already know the numbers can be a little skewed in grown-ups: An athlete with heavy muscles, for example, might come out as "obese" in terms of BMI. For kids, the numbers are even harder to interpret.

If your child is in the middle range of BMI—remember, that's most of the population—then the statistic tells you nothing at all. He might be fat or he might be lean; there's no way to quantify that without doing another test, like a tissue scan. Among the kids who are classified as "overweight"—in the 85^th percentile or above—a full four-fifths turn out to be in the normal range for body fat. The BMI percentiles are most meaningful at the heaviest extreme: If your child is in the 95^th percentile or above, there's a good chance that he's actually fat. (You might think that a mother can tell whether her child is fat or lean, just by looking at him. That's not always the case.)

These discrepancies suggest that we needn't wring our hands over the children who fall between the 85^th and 94^th percentiles. In fact, we only started classifying those kids as "overweight" in the past few years. (Previously, they were described as being "at risk for overweight.") When Michelle Obama says that one in three kids is now overweight or obese, she's lumping together two distinct populations with different risk profiles. In February, she conflated the numbers even further with a slip of the tongue. In an interview with Matt Lauer on the Today Show, she said that "one in three kids are obese in this nation." This little mistake—sure to be repeated in the coming months—exaggerates the magnitude of the crisis twofold.

Here's another stat we'll see again and again in discussions of childhood obesity. The childhood obesity rate has been defined such that it was 5 percent in the 1960s and 1970s. Today, it stands at an all-time high of 17 percent. But that doesn't mean obesity rates are still skyrocketing or that American kids will keep getting fatter unless we take drastic action to avert a crisis. In fact, the most recent data from the CDC—published in January—suggest that the numbers have started to level off.

Among adults, obesity rates went way up in the 1980s and 1990s, then reached a plateau among women at around 35 percent. (They seem to be doing the same for men, at 32 percent.) The data for children and teenagers tell a similar story, of a rapid rise through the end of the Clinton years and then a stable signal over the last decade. (There are some small exceptions: Rates have increased, in the aughts, among some of the very heaviest boys.)

Meanwhile, newborns seem to be getting smaller. A study published last month by Sara Donahue and Emily Oken looked at the birth weights of 37 million full-term babies between 1990 and 2005 and found that the numbers are declining. Oken, a professor at the Harvard Medical School, suggests that this surprising reversal might be a leading indicator when it comes to changes in body size overall. (Previous research has shown a linear relationship between birth weight and adult size, as measured by BMI.)

Kate Harding highlighted the trend data (and made many other excellent points) in an essay for Salon last month. The first lady is "exaggerating the crisis," she says, by failing to mention that obesity numbers are flattening out. We can't ignore the fact that one-sixth of American children are obese, but there's no reason to think the epidemic is getting worse.

This one is the real canard. There's no solid evidence whatsoever to support the claim that our children's lives are going to be shorter than our own.

Life expectancy in the United States has increased by more than 28 years since 1900. (It's now up to 77.9.) In 2003, the chief actuary of the Social Security Administration testified before Congress and made what some demographers considered to be a conservative projection—that the average lifespan will have risen to 79.4 years by the year 2050. If he's right, it will take America a full half-century just to match the public health standards that already exist in countries like France and Japan.

By '03, though, public health officials here and abroad had begun to sound the alarm bells for a decreasing expected lifespan. In 2004, U.S. Surgeon General Richard Carmona suggested to Congress that we'd reached an ominous threshold: The youngest generation was on track to lead shorter lives than their parents.

That gloomy perspective got some support the following year, with the publication of a report on the effects of obesity on lifespan in the New England Journal of Medicine. The authors concluded that rising rates of type 2 diabetes, heart disease, and cancer would soon overwhelm whatever advances in medical technology could be expected in the near future.

Not everyone agreed, of course. A more recent NEJM study, published in December, concluded that the life-shortening effects of obesity will be offset, in part, by the decreasing rate of tobacco use. Add in whatever improvements in medicine and public health might reasonably be expected, and the authors suppose that "life expectancy will continue to rise, but less rapidly than it otherwise would."

Still, the most extreme form of the prediction—that our longevity has already begun to decline—has been repeated by just about everyone who's involved in the war on obesity. (A few weeks ago, celebrity chef Jamie Oliver trotted out the distortion while accepting a $100,000 prize for his work on the obesity issue.) Never mind that it hasn't been substantiated, and won't be. As the first lady's campaign gathers steam, we'll hear a lot more about our fat kids and their shrinking life spans.