Researchers led by Jeanne Van Cleave, a doctor at Massachusetts General Hospital for Children in Boston, looked at the prevalence of conditions that lasted a year or longer in three groups of children, starting with a first cohort of more than 2,000 kids in 1988.

That group was tracked for six years, after which a second group was studied between 1994-2000 and finally a third group from 2000- 2006.

Mothers of the children were asked whether their kids had any “physical, emotional or mental condition that prevented him or her from attending school regularly, doing regular school work or doing usual childhood activities, or that required frequent attention or treatment from a doctor or other health professional.”

The information gathered was classified into one of four categories of chronic condition: asthma, behavior or learning disorders, obesity and other physical conditions.

“We found that prevalence of a chronic condition at any point during the study period was very high and increased over time,” the authors of the study, published in the Journal of the American Medical Association, said.

“Many factors may have contributed, including environmental changes, which may affect rates of chronic respiratory conditions and obesity,” and greater access to health care for children during the study period, which would have boosted diagnoses of childhood chronic conditions, the study says.

Reports of all chronic conditions, including the much-talked-about childhood obesity, rose from just under 13 percent at the end of the six-year follow-up for the first group of children to 26.6 percent in 2006, the study shows.

The obesity rate rose from 13.3 percent at the end of the first study group, in 1994, to nearly 16 percent at the end of the third cohort in 2006.

In the third and last group the researchers looked at, 51.5 percent of eight- to 14-year-olds “at one point in the six-year study period reported a chronic condition compared with 27.8 percent in cohort one,” the study says.

But unlike chronic conditions in adults, the childhood conditions were not necessarily long-lasting, the study said.

More than half of children who showed asthma-like wheezing before they were four years old had stopped having breathing difficulties by age six, and children with certain behavior disorders overcame them within a year.

The study also confirms what other recent research has shown: that obesity in the United States has reached a plateau.

There were fewer new cases of obese children reported in the third group of children — between 2000-2006 — than in the second group, a finding “consistent with previous reports of flattening childhood obesity rates in recent years,” the study says.