ADHD Rates and Birth Month

Published - Written by Amanda Fernandes, MD

In 1902, British pediatrician Dr. George Still first described a case series of 43 children of normal intelligence with a “defect of moral control.” In 1937, Dr. Charles Bradley noted an unexpected side effect — an improvement in school performance and social interactions — after administering a drug for headaches that contained amphetamine to children at a school for behavioral disorders. Decades later and after several name changes, attention deficit–hyperactivity disorder (ADHD) is the most common behavioral disorder in children.

According to the Diagnostic and Statistical Manual (DSM-5), the diagnosis of ADHD is based on symptoms of inattention and/or hyperactivity-impulsivity for a minimum of 6 months. These symptoms must be present in two or more settings and interfere with social and academic interactions. In 2016, the CDC estimated that 1 in 10 children aged 2–17 years in the U.S. had ever been diagnosed with ADHD. The rising rate of ADHD diagnoses is concerning.

In this week’s issue of NEJM, Layton and colleagues used a large insurance database to compare rates of ADHD diagnosis between children born in August and children born in September in states that did and did not require that children reach their 5th birthday by September 1 to start kindergarten. They hypothesized that there would be differences in behavior between the younger children born in August and children born in September in states with a September 1 cutoff because of the12-month age span within the grade.

The study included approximately 400,000 children born between 2007 and 2009 and entering kindergarten between 2012 to 2014. A diagnosis of ADHD (the primary outcome) was based on International Classification of Diseases, 9th Revision [ICD-9] codes. Prescription records were also examined to compare treatment differences. In the 18 states with a September 1 cutoff, the rate of ADHD diagnosis was 34% higher among children born in August, compared with children born in September who started kindergarten a year later, and the rate of ADHD treatment was 32% higher. Rate differences were not seen in states without the September 1 cutoff.

Dr. Jenny Radesky notes in NEJM Journal Watch, “This study illustrates a phenomenon we often see in children (particularly boys) with summer birthdays entering kindergarten whose teachers may perceive them as having more behavioral or academic difficulties.” These problems may be more related to the younger age of the child than ADHD. For children born in August, delaying the start of kindergarten may be more beneficial than parents and physicians realize.

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 Amanda Fernandes is a 2018-2019 NEJM editorial fellow.