(CNN) - Higher rates of ADHD diagnoses have been linked to babies born in a certain month in certain US states.
In states where children must turn 5 by Sept. 1 in order to start kindergarten, are more likely than children in other states to be diagnosed and treated for ADHD, according to a study published in the New England Journal of Medicine.
Many couples try to plan the perfect month to get pregnant and a new study found Aug. may not be the best month to give birth.
There are 21 states with this cutoff date, according to the Education Commission of The States.
Meaning kids born in Aug. in these states are usually the youngest in their classrooms, which researchers say could be part of the problem.
Health experts say the study confirms on a larger scale, what some other studies have shown on smaller ones.
Dr. Dimitri Christakis, director of the Center for Child Health, Behavior and Development at Seattle Children’s Research Institute, wrote in an email to CNN:
"This study confirms on a larger scale what other studies have shown on a smaller one. It confirms that there may be developmental confounders in the diagnosis of the condition and that the relative immaturity of young children's brains can make those that are merely younger at school entry demonstrate behaviors consistent with ADHD," Christakis wrote.
But the study doesn’t account for how kids were diagnosed.
Or if their physicians were following the right protocols to properly diagnose them for ADHD.