Babies with small birthweights are at an increased risk of depression in later life, research suggests.
Events while in the womb may be important
Newborns weighing under 5.5lbs (2.5kg) were 50% more likely to have anxiety and depression as adults, a British Journal of Psychiatry study found.
Small babies are known to be prone to certain diseases and learning difficulties, and mounting evidence now suggests they risk mood disorders too.
The Bristol University team believe harm while in the womb may be a factor.
Taking factors such as the individual's IQ, and whether or not they had behavioural problems as a child into account did not alter the findings.
Nor did factors such as social class, or how old their mother had been when they had given birth to them.
Lead researcher Dr Nicola Wiles said: "It was a direct effect, so we think early factors happening before birth might be important."
Her team at Bristol, working with colleagues at the London School of Hygiene and Tropical Medicine, used information from over 5,572 participants from the Aberdeen 'Children of the 1950s' study.
They compared rates of depression when the participants had reached the ages of 45 to 51 with birthweight, plus their mental and behavioural development as children.
Dr Wiles said that the trend between low birth rate and depression in adulthood might be down to restricted growth in the womb impairing brain development in some way.
She said: "What we need to do is understand what is going on in terms of the biological mechanism.
"We know that a lot of the brain growth occurs in the womb. It may be a delayed effect that we are seeing."
However, she emphasised that people should not be alarmed by the findings and that there were many other causes of depression and this was merely another factor to consider.
"We do not want to spread panic with mothers who fear they may give birth to a smaller child," she said.
There have been previous reports that birthweight is linked with depression in later life.
A study the Medical Research Council, published in 2004 and which involved over 5,000 participants, found a similar trend.
The authors of MRC study also suggested that it might be related to stunted growth in the womb.
They said: "Stress at a critical time during foetal development may increase susceptibility to this condition."
They said animal studies showed exposure to various stressors during pregnancy results in lower birth weights and physiological features that were very similar to those seen in people with depression.
A spokesman from BLISS, the preamture baby charity, said they were not surprised by the findings.