Youngest in class at higher risk of mental health problems

A recent study by the Norwegian University of Science and Technology (NTNU) has found that children born in October, November or December are statistically more often identified as having a mental health diagnosis than their classmates born earlier in the year. The findings apply to both boys and girls, and regardless of whether they were born full term or prematurely.

Extensive research material

The researchers have followed over one million Norwegians aged 4 to 17 years (all born between 1991 and 2012) through Norwegian health registries.

The aim of the study was to identify what are known as ‘relative age effects’. In other words, whether children and adolescents born late in the year are more frequently diagnosed with mental health disorders than their peers born early in the year (January, February and March).

“Our findings show that the youngest members of a school class tend to be diagnosed with a mental illness more frequently than the oldest,” said Christine Strand Bachmann, a PhD research fellow at the Norwegian University of Science and Technology (NTNU’s) Department of Public Health and Nursing.

“This is most obvious with regard to ADHD, where we saw an increase in incidence of 20-80 per cent for the youngest class members, depending on whether the children were born full term or prematurely.”

The researchers found the same trend for ‘other neuropsychiatric disorders’. These include delayed developments in areas such as language, academic skills and motor skills.

The study has been published in BMJ Peadiatrics Open.

Additional risks for premature girls

In addition, the youngest premature girls were at a significantly greater risk of being diagnosed with emotional disorders, such as anxiety, depression and adaptation disorders, compared with the oldest premature girls in the same year group.

“We know that children and adolescents born prematurely are already more vulnerable to poor mental and social health compared with children and young people born full term. For those born prematurely, the risks associated with being born late in the year comes in addition to this vulnerability.

“We believe that these findings, which show an increase in the number of psychological diagnoses for the youngest class members, can partly be linked to the way in which we organize our education system. The school system is unable to adequately provide for children with normal, but more immature behaviour. Possible solutions include flexible school start dates or additional support.”

In addition to being a researcher at NTNU, Christine Strand Bachmann is also a consultant at the Neonatal Intensive Care Unit, Children and Adolescent Medicine Department, St. Olavs Hospital.

Reference:
Christine Strand Bachmann, Kari Risnes, Johan Håkon Bjørngaard et al: Relative age as a risk factor for psychiatric diagnoses in children born preterm and to term: a cohort study BMJ Pediatrics Open, 5 April 2025