The research found that children born full term but weighing less than 5.5 lbs (almost 3% of the total sample) had a 50% increased risk of psychological distress in later life. This remained the case after taking into account potential confounding factors, such as the father's social class, maternal age and adult marital status.
Until now it has been unclear whether the effect of low birth weight on common mental health problems in later life is direct, or is affected by childhood factors, such as IQ or behavioural problems.
Dr Nicola Wiles, from Bristol University and lead author on the study, commented: "The findings suggest that low birth weight at full term has a direct effect on adult mental health, rather than simply reflecting a pathway through childhood cognition and/or behaviour. This is an interesting finding that requires replication in other studies but suggests that early factors, before birth, might be important in increasing vulnerability to depression in adult life".
This study used information on 5572 participants in the Aberdeen Children of the 1950s study for which data collection was funded by the Medical Research Council. The researchers from the University of Bristol and the London School of Hygiene and Tropical Medicine examined the association between birth weight for gestational age and later adult psychological problems, taking into account cognition and behavioural problems in childhood.
No increase in risk was found in those of low birth weight who were born early, before 38 weeks. Similarly, pre-term delivery was not associated with an increased risk of psychological distress in adulthood.
As found in previous studies, low birth weight was associated with an increased risk of cognitive deficit (having an IQ of less than 100) at the age of seven, and with childhood behavioural disorder. This effect was observed among those born early as well as those born at term.
IQ of less than 100 at age seven was associated with an increased risk of psychological distress in adulthood. But taking into account IQ and behavioural factors did not alter the strength of the association between low birth weight at full term and adult psychological distress.
Low birth weight for gestational age is a marker for impaired foetal growth. The observed association with adult psychological distress provides further evidence for the theory that common mental health problems in adulthood may be due to impaired neuro-development, as has been suggested in schizophrenia.
Further work is needed to explore the biological mechanism underlying this relationship.