Our primary objective was to investigate whether calendar month of birth is associated with the risk of receiving ADHD medication or an ADHD diagnosis in Norway. The large population size and the inclusion of sibling identifiers enabled us to apply an additional sibling design to control for unknown family factors. Furthermore, the long follow-up enabled us to investigate these effects by school grade.

Several studies have reported that month of birth is associated with the risk of being diagnosed with ADHD or prescribed ADHD medications [ 7 – 17 ]. Many factors potentially associated with childhood neurodevelopmental disorders, ranging from maternal socioeconomic status to fetal and early life infections, could vary with season of birth. However, associations between month of birth and ADHD have mainly been attributed to age at school enrolment and relative age in a class [ 7 – 17 ]. Children born just prior to the cut-off date for school enrolment are up to 12 months younger than their oldest classmates, and may therefore be relatively less mature intellectually, emotionally and physically. Relative age in grade may be particularly important during the first years in school, as mental and physical growth can develop rapidly over a few months at this age. Interestingly, studies from Denmark, where school enrolment dates are less stringently enforced, have not reported associations between birth month and ADHD [ 18 , 19 ].

Attention-deficit/hyperactivity disorder (ADHD) is one of the most common neurobehavioral disorders among children and adolescents, estimated to affect 3–6% of children [ 1 ]. The diagnosis is clinical and criteria-based, and characterized by the three core features of ADHD: hyperactivity, impulsivity and/or inattention. The core features should persist over time and impair functioning across multiple settings, and be more prominent than expected given the child’s age [ 2 ]. ADHD is managed by behavioural therapy and pharmacological treatment, mainly with psychostimulants and atomoxetine [ 3 ]. There is a substantial variation in prevalence of diagnosis and treatment worldwide [ 4 ], between the Nordic countries [ 5 ] and between counties within Norway [ 6 ]. This observed variation is, at least partially, likely to be explained by other factors than regional differences in the presence of ADHD as a clinical entity.

In the final analytical approach, we conducted a cross-sectional time-series analysis of the risk of receiving ADHD medication while attending a given grade, with month of birth as exposure variable. The outcome was dispensed medication during a particular school grade (starting in August and ending in July the next year), irrespective of any previous dispensed medication. We estimated odds ratios (ORs) for receiving medication during a given school grade in panel-data models with grade as the time variable and autoregressive correlation structure. These analyses were also stratified by sex and adjusted for year of birth and parental education, and included an interaction term between grade and month of birth. Results are presented as estimated marginal means for proportions with ADHD medication.

In a supplementary analysis, we used a stricter definition of ADHD treatment whereby children were classified as receiving ADHD treatment only if they received ADHD medications at least twice with a minimum of 30 days between dispensing dates. We further restricted the dataset to siblings (defined by the mother’s unique study allocation number) with discordant use of ADHD medication, and applied a stratified Cox model to control for shared family factors in the analysis of risk of ADHD medication by birth month [ 22 ]. This latter analysis included data for both boys and girls, and we adjusted for year of birth, sex and birth order.

In three separate analyses by outcome, we followed children from their sixth birthday until the month of the outcome (dispensed medication, specialist diagnosis or GP diagnosis), death, emigration, or end of follow-up, whichever came first. To avoid differential observation time due to month of birth, end of follow-up was defined according to month of birth; that is, children born in January were followed until 31 January 2014, while children born in February were followed until 28 February 2014, and so on. We initially analysed the data by Kaplan–Meier failure curves stratified by sex and month of birth. We then applied Cox proportional hazards regression analyses with number of days from age six as the time metric and month of birth as exposure. We adjusted for year of birth and parental education. Birth month was categorized as January–March (reference category), April–June, July–September and October–December. Inspection of log-minus-log survival curves did not indicate violation of the proportional hazard assumption. Data were analysed separately for boys and girls.

We obtained information on ADHD diagnoses from two separate sources: the Norwegian Patient Registry (NPR) and the Norwegian Directorate of Health. The NPR holds information from specialist healthcare services from 2008 onwards. We defined ADHD by the ICD-10 code F90 ‘Hyperkinetic disorder’. The Norwegian Directorate of Health reimbursement database for GPs holds information on consultations in primary healthcare from 2006 onwards. We defined ADHD diagnosis by the International Classification of Primary Care 2nd edition (ICPC-2) code P81 ‘Hyperkinetic disorder’.

Our main outcome was use of medications for ADHD. This information was retrieved from the Norwegian Prescription Database (NorPD), which contains data on all prescribed medications dispensed from pharmacies to individuals from 2004 onwards. We retrieved data on all dispensed medications approved in Norway for treatment of ADHD (methylphenidate, atomoxetine, racemic amphetamine, dexamphetamine and lisdexamphetamine) [ 21 ].

The study population included all children born in Norway during 1998–2006 registered in the National Registry as resident in Norway at age six ( N =509,827). Data from the National Registry included information on sex and dates of birth, emigration or death. We retrieved information on dispensed ADHD medications, ADHD diagnoses and parental educational level by linking the National Registry with four national registers based on the personal identity number. Unique study allocation numbers replaced all personal identity numbers. The National Registry file included study allocation numbers for parents in addition to the child’s number. The study was approved by the Norwegian Data Protection Authority (approval number 10/00910-12) and the Regional Committee for Medical and Health Research Ethics (approval number 2010/2583).

The Norwegian educational system is government-funded, and attending grades 1 through 10 is free and compulsory by law. Children enrol in elementary school in the calendar year they turn six years old. Thus, the oldest children in a grade are born in January and the youngest in December. Early or delayed enrolment is rare and only permitted under special circumstances, after evaluation by an appropriate expert and approval by the child’s parents and the municipality. The school year runs from mid-August until mid-June.

The Norwegian healthcare system is financed by the government through taxes and covers all citizens. Inpatient and outpatient healthcare, as well as reimbursable medications, is free of charge for children and adolescents. Access to specialist healthcare services requires referral from a general practitioner (GP) [ 20 ]. Diagnostic criteria from either The International Classification of Diseases 10th Revision (ICD-10) or The Diagnostic and Statistical Manual of Mental Disorders (DSM) can be used in the diagnostic examination of ADHD [ 21 ]. GPs are usually responsible for regular follow-up of children diagnosed with ADHD in specialist healthcare. Until 1 January 2014, initiation of treatment with stimulants was restricted to physicians with a medical specialty in either child and adolescent psychiatry, psychiatry, neurology or paediatrics. Physicians without the appropriate medical specialty could apply for authorization to prescribe medication to individual patients. This restriction was liberalized from 2014 onwards, and GPs may now maintain stimulant treatment initiated in specialist healthcare [ 21 ].

Finally, we analysed the risk of receiving ADHD medication while attending a given grade in a cross-sectional time-series model. Figure 3 shows that from grade 3 through grade 9, the children born later in the year were at higher risk of receiving ADHD medication. The proportions increased by grade level and differences were most pronounced in the higher grades. The confidence intervals in Figure 3 give an indication of statistically significant differences between groups. For instance, when we compared boys born in October–December with boys born in January–March, p -values from contrasts of predictive margins were <.001 from grade 3 onwards, while there were no statistically significant differences between boys born in April–June and January–March in any grade. Similarly, differences between girls born in October–December and January–March were significant ( p <.001) from grade 3 onwards. Among girls, differences between those born in April–June and January–March were, however, also statistically significant (grade 3: p =.013, grade 4: p =.001, grade 5 onwards: p <.001).

We repeated the Cox analyses using ADHD diagnosis from specialist healthcare or from primary healthcare (GPs) as outcome. The pattern, with increasing risk for children born later in the year compared with children born earlier in the year, was consistent with the findings for dispensed ADHD medications ( Table II and Figure 2 ).

In the analysis applying a stricter definition for ADHD treatment (dispensed medication at least twice), risk estimates were very similar, as only 869 of the 15,717 children with ADHD medication were reclassified. In analyses restricted to siblings within the study population (17,017 children among 7,690 mothers), the adjusted hazard ratio was 1.3 (1.1–1.4) for children born in April–June, 1.5 (1.4–1.7) for July–September and 1.7 (1.5–1.9) for October–December, compared with the siblings born in January–March.

At end of follow-up, the proportion of boys born in October–December having received ADHD medication was 5.3% compared with 3.7% of those born in January–March, while for girls the corresponding numbers were 2.2% versus 1.3% ( Table I ). The patterns for ADHD diagnoses were similar. Figure 1 shows Kaplan-Meier failure curves for dispensed ADHD medication by month of birth. From an early age, boys born during July–September and October–December had a higher risk of receiving ADHD medication than boys born earlier in the year, and this pattern persisted up to age 14 (end of follow-up). The same pattern was observed for girls. The risk estimates from Cox analyses increased by month of birth for both sexes ( Table II and Figure 2 ). The adjusted hazard ratio was 1.4 (95% confidence interval: 1.4–1.5) for boys born in October–December compared with boys born in January–March. The corresponding adjusted hazard ratio for girls born in October–December was 1.8 (1.7–2.0). Adjustment for parental education and year of birth did not influence the estimates.

The study population included 509,827 children born in Norway during 1998–2006 ( Table I ). In total, 15,717 children (4.4% of boys and 1.7% of girls) received ADHD medication at least once, while slightly higher proportions received a diagnosis from specialists (4.8% of boys and 1.9% of girls) and primary healthcare GPs (4.5% of boys and 1.8% of girls), respectively.

Discussion

In this register-based study from Norway, we have demonstrated that children born late in the year more often receive ADHD medications than children born early in the year. The risk of receiving medications increased by month of birth for both boys and girls. This pattern of increased risk was consistent from grade 3 through grade 9, with most pronounced differences in the higher grades. Analyses with diagnoses of ADHD from either specialists or GPs as outcome gave similar results. Sibling comparisons supported the main findings.

Variation through the year in environmental risk factors for developing ADHD could potentially contribute to a difference in incidence of ADHD by season of birth. However, the observed association with birth month seems unlikely to be explained by a biological mechanism, as conceivable environmental risk factors are not likely to change substantially and abruptly between the two consecutive months December and January. Results also remained robust after adjustment for parental education and year of birth, and in sibling analyses. Furthermore, the point estimates increased similarly and progressively by birth month for both medication and diagnosis as outcome.

Previous studies have reported an increased risk of receiving a diagnosis or medication for ADHD for children youngest in class across several countries with quite different ADHD treatment practices and healthcare systems (the USA, Canada, Netherlands, Germany, Iceland, Sweden, Spain, Israel, Australia and Taiwan) [7–17]. One proposed explanation is a relative age effect within class, whereby children who are youngest among their peers on average are less mature intellectually, emotionally and physically. The risk pattern presented in these previous studies aligns with the cut-off date for school enrolment in the given country or region rather than the season of birth, which further supports the relative age hypothesis. For instance, children born in August are the youngest in class in Taiwan and were at significantly higher risk of receiving ADHD medications and diagnosis [15], and studies across several states in the USA with different cut-off dates for kindergarten enrolment reported that the risk pattern aligned with these cut-off dates [7,8]. Interestingly, the study from Sweden [12] found an association between birth month and ADHD diagnosis and medication, but no such association with self- or parent-reported symptoms of ADHD in a subsample of nine-year-olds.

Contrary to the findings in the present study, which are in line with most other studies in the field [7–17], no clear association with month of birth has been reported from Denmark, at least not in the most recent years [18, 23]. Pottegård et al. argue that this could be due to the common practice in Denmark of allowing children to delay school enrolment by one year, especially for children born closest to the school enrolment cut-off date [23]. In two other Danish studies, Dalsgaard et al. pointed to the relatively restrictive use of ADHD medications in Denmark, and that specialists are ultimately in charge of diagnosing ADHD and for initiation of pharmacological treatment [18]. However, Norway also has a relatively restrictive system whereby specialists diagnose ADHD and initiate pharmacological treatment, and GPs were only recently allowed to maintain treatment with stimulants [21].

Diagnostic guidelines for ADHD suggest that symptoms of inattention, hyperactivity and impulsivity should be clearly more pronounced than expected for the child’s age [21]. Thus, the decision on whether to refer children to specialist healthcare, as well as the diagnostic examination itself, could partly be based on judgments of a child’s behaviour and achievements relative to older peers. Large and unexplained geographical variations within Norway have previously been observed for ADHD diagnoses [6]. Such findings, together with the present study, may challenge perceptions and management of ADHD, as well as age at school enrolment.

Our study showed that the birth month effect was sustained throughout childhood and into adolescence, and we even observed stronger effects in higher grades. Interestingly, a recent Norwegian study has shown that the youngest children in class had lower probability of having completed high school at age 19, were less likely to have enrolled in college by age 25, and had lower earnings at age 30, indicating that the drawback of being youngest in class even persists into adulthood [24].

The main strength of our study is the population-based design utilizing several national health registers with mandatory reporting covering the entire population. We were able to capture all children with ADHD medication and children with an ADHD diagnosis from specialist and primary healthcare. The robustness of our findings using information on ADHD from three independent registries and different analytical approaches, including restriction to siblings, strengthens the validity of our results.

One limitation was that the study population also included some children with early or delayed enrolment into school, who therefore did not attend their age-assigned grade level. However, grades 1 through 10 are compulsory by law, and delayed enrolment is permitted only under special circumstances. Among children born between 1969 and 1991, the proportion with delayed entry in school decreased with increasing birth year, with only 1.5% of boys and 0.9% of girls born in 1991 enrolling late [24]. Thus, the frequency of early and delayed school entry is most likely to be very low, and we regard birth month as a good predictor of relative age in class. Another limitation is that we were not able to validate the diagnoses and could therefore not confirm or disprove whether the clinical picture was consistent with ADHD. However, the research question was not whether children who are born later in the year are at higher risk for ADHD as a clinical entity, but whether they are more prone to being diagnosed and/or treated for this disorder. In this context, the risk for bias due to misclassification is minimal. Specific formulations of drugs used to treat ADHD can also be used for sleep disorders such as narcolepsy, but these conditions are extremely rare [25]. Follow-up from age six was complete for all children in the study population, and ADHD is rarely diagnosed or treated at preschool age. Data on primary and specialist healthcare diagnoses are captured from 2006 and 2008 onwards, respectively. Information on first diagnosis is likely to be missing for a larger proportion of children than information on the first dispensed medication, which is available from 2004. However, the results based on medication and diagnoses were highly consistent.