Background It is unclear whether the effect of low birth weight on common affective disorders in later life is direct or mediated through childhood factors.

Aims To determine whether birth weight has a direct effect on psychological distress in adulthood not mediated by childhood IQ or behavioural problems.

Method Participants (n=5572) of the Aberdeen Children of the 1950s study had data on birth weight for gestational age and adult psychological distress. Logistic regression was used to examine the association between these factors, with adjustment for confounders and potential childhood mediators.

Results Children born full term but weighing less than 5.5 lb had increased odds of psychological distress in later life after adjustment for potential confounders (OR=1.49, 95% CI 1.01-2.20). Further adjustment for childhood IQ and behaviour did not attenuate the association. A 1 s.d. decrease in birth weight for gestational age was associated with a 4% increased odds of psychological distress in adulthood (OR=1.04, 95% CI 0.97-1.12).

Conclusions Low birth weight for gestational age, particularly at term, was associated with adult psychological distress. This was not mediated by childhood factors, suggesting a direct effect of early life factors on adult mental health. A neurodevelopmental pathway may therefore be implicated.

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