(Reuters Health) - Being one of the youngest in a kindergarten class dramatically increases the odds that a child will be diagnosed and treated for attention deficit-hyperactivity disorder(ADHD), a large new U.S. study has concluded.

The findings, reported in the New England Journal of Medicine, provide evidence that some children are being diagnosed with ADHD not because they have the condition, but because they are less mature than others in their class, who can be up to 11 months older.

Doctors were able to see the trend by looking at diagnosis and treatment rates based on the birth dates and health insurance claims records of more than 400,000 children born from 2007 through 2009.

When the cutoff for attending kindergarten was having a birth date before September 1, the odds of being labeled hyperactive were 34 percent higher for the youngest children - those born in August - than for the oldest children, those born the previous September.

“Each week after that Sept. 1 cutoff, they’re more likely to be diagnosed with ADHD,” lead study author Timothy Layton told Reuters Health in a telephone interview. “We were able to show it in a very clean and transparent way.”

When the researchers looked at children in states that didn’t have the September 1 cutoff date for starting kindergarten, kids born in August were not more likely to be diagnosed with ADHD.

Because smaller studies published years ago have suggested the same thing, “It’s surprising that this phenomenon still even exists,” said the study’s senior author Dr. Anupam Jena, a professor of healthcare policy and medicine at Harvard Medical School in Boston.

His advice to parents: “If a doctor or teacher is considering a diagnosis of ADHD and the child is young for their grade, they might pause and say, let’s see how this child develops over the next 6 months or a year before we finalize the diagnosis and start to make recommendations for treatment.”

The results come at a time when more than 1 in 20 U.S. children aged 2 to 17 years are taking medicine to treat ADHD, and the diagnosis is becoming increasingly prevalent.

There is also a longstanding concern that some children may be getting ADHD treatment even though their attentiveness is appropriate for their age.

The researchers found that out of every 10,000 children born in August, 85.1 were diagnosed with ADHD compared to 63.6 kids born in September.

Gender seemed to also play a role. Among boys, the odds of being diagnosed with ADHD were 64 percent higher if they were born in August instead of September. Boys are more likely to be given the diagnosis in general.

A 28 percent increase was seen among girls born in August in states with the September cutoff, but Layton noted that this increase was not large enough to be statistically significant “because not as many girls are getting diagnosed” to begin with.

Not every child diagnosed with ADHD was treated, but the data suggest “the children born in August who received a diagnosis of ADHD because of their August birthdays received more intensive treatment than the average child with ADHD who was born in September,” Layton’s team writes in their report.

The treatment rate per 10,000 August-born children was 52.9, compared with 40.4 per 10,000 among youngsters with a September birthday.

Among the study’s limitations is that it did not double-check each ADHD diagnosis, so it is possible that the August-born children given that diagnosis were correctly categorized and September-born children with ADHD were missed. Because of the database used, only the children of parents who had employer-provided health insurance coverage were included; children covered by Medicaid or whose parents had no health insurance at all were not tracked.

SOURCE: bit.ly/2BuVlQR The New England Journal of Medicine, online November 28, 2018.