Influence of birth month on the probability of Western Australian children being treated for ADHD

Whitely and colleagues1 reported that children born in June were more likely to receive treatment for attention deficit/hyperactivity disorder (ADHD) than children born in July. It is unfortunate that they did not measure the year of school intake, and rather made the assumption that all children entered school at the recommended age. The authors concluded that “there are significant concerns about the validity of ADHD as a diagnosis,” which is a large leap not substantiated by the data presented.

We are conducting Australia’s first community-based longitudinal study of children with (n = 179) and without ADHD (n = 212), the Children’s Attention Project.2,3 Our design is ideal for examining this research question as children were all recruited across one year of school, enabling us to accurately categorise children as early or late starters. We rigorously assessed for ADHD (ie, via parent and teacher surveys and diagnostic interviews) and also recorded the use of ADHD medications.

To investigate the same question within our cohort, we defined early starters as those children with birth months in February, March or April (age at entry, 4 years 9 months to 4 years 11 months; n = 46) and late starters as those children born in May, June or July (age at entry, 5 years 6 months to 5 years 8 months; n = 123). We found no relationship between being an early or late starter and meeting the criteria for ADHD at either age 7 years or age 10 years. Our sample size for these analyses was small because most children with ADHD were born outside the months encapsulating early and late start date definitions (n = 112; 63% of our ADHD sample). Only nine children were prescribed medication across the early and late starter definitions — one early starter and eight late starters.

In conclusion, our data show that when school commencement year is considered, there is little evidence to support early versus late school starter status as a predictor of ADHD and treatment in Victoria. The overall rate of ADHD medication prescribing for children aged 6–15 years reported by Whitely and colleagues was 1.9%.1 ADHD has a prevalence of about 5%,4 thus, these data provide further reassurance that the rates of prescribing of ADHD medications in Australia remain moderate.