Small babies have a higher risk of depression and anxiety in later life, say UK researchers.
Low birth weight is linked to a risk of poor health
Delays in developmental milestones, such as walking, are also linked to poorer mental health in adulthood, a study of 4,600 people suggests.
The team, writing in Biological Psychiatry, said low birth weight was an indicator of stress in the womb, which can adversely effect the foetus.
Low birth weight is associated with a range of long-term health problems.
The Medical Research Council funded study used data from a group of people born in Great Britain in 1946 who had been assessed for symptoms of depression and anxiety at 13, 15, 36, 43 and 53 years of age.
Heavier babies were less likely to suffer symptoms of anxiety and depression.
They also found the lower the birth weight, the greater the likelihood for repeated or long-term problems in adulthood.
The researchers could not look at premature birth as a factor as this was not recorded at the time but they did take into account social circumstances and stressful events during childhood.
People who had worse mental health throughout their lives were also found to have reached developmental milestones - like standing and walking for the first time - later in life than those who had better mental health.
Study leader Dr Ian Colman, who was based at the University of Cambridge when the study was done, said even people who had mild or moderate symptoms of depression or anxiety were smaller babies than those who had better mental health.
But he stressed that not all small babies will experience poor mental health in the future.
"Being born small isn't necessarily a problem," he said.
"It is a problem if you were born small because of adverse conditions in the womb - and low birth weight is what we looked at in this study because it is considered a marker of stress in the womb.
"When a mother is really stressed, blood flow to the uterus is restricted and the foetus gets fewer nutrients, which tends to lead to lower birth weight."
He added that stress hormones passing through the placenta to the foetus may affect neurodevelopment and stress response.
The researchers theory is that the foetal brain could be incorrectly programmed leading to people who were smaller babies being more likely to become depressed or anxious when faced with stressful events.
Dr Colman said the take-home message from the study was that we should take "better care" of pregnant women to reduce the stress they are under.
Dr Virginia Beckett, consultant obstetrician in Bradford and spokesperson for the Royal College of Obstetricians and Gynaecologists said it was hard to pick cause and effect out of such studies as there were many factors which had an impact on depression and anxiety, as well as on birth weight.
"But it's an interesting trend.
"Studies like this are supportive of our argument that we have to look after mums adequately as the impact of maternal health on children is far more than we realise.
"The health of pregnant women needs to be taken much more seriously by us as a society."
Professor Andrew Shennan, spokesperson for Tommy's, the baby charity, said the findings were important but a firm conclusion could not be drawn.
"It could be just as likely that whatever makes you small makes it more likely to become depressed or anxious in later life."