Babies born to women who suffer post-natal depression are more likely to become violent and aggressive children, research suggests.
A very important time of life
Cardiff University researchers followed children up to the age of 11 from 122 families in urban south London.
They found boys with mothers who were depressed three months after birth were more likely to get into fights and be suspended from school.
The research is published in the journal Developmental Psychology.
The Cardiff team found that children were most likely to be badly behaved if their mothers had suffered repeated bouts of depression.
They interviewed mothers during pregnancy, three months after the birth, and when the child was one, four and 11 years old.
Most children in the study were not violent, but the group who were most likely to show sign of problem behaviour were boys whose mothers were depressed three months after birth, and at least once at a later date.
Girls seemed to be less vulnerable.
The researchers believe that the poor behaviour is in part caused by children's inability to control their emotions, or to pay proper attention.
However, they are uncertain why this should be linked to their mother's difficulties with post-natal depression.
They say it is possible that the mother's hormones may have a direct impact on their child's development. A separate study found that women who were more anxious in pregnancy were more likely to have boys prone to behavioural problems at the age of five.
Alternatively, mothers may become depressed because they struggle to cope with a baby who is already demonstrating signs of a difficult temperament that then goes on to cause trouble as they grow.
Heather Welford, an expert in post-natal depression at the National Childbirth Trust, told BBC News Online previous studies had produced similar findings.
She said: "While mothers with post-natal depression care for their babies perfectly well in a physical way, it does seem that they tend to have difficulty communicating with their baby, either by talking or singing to them, or by playing with them.
"This may lead to a communication deficit in the first year of the baby's life."
Ms Welford said many women did not recognise the fact that they had post-natal depression, and even those that did were not always easily able to find help.
She said: "This study underlines the fact that mothers with post-natal depression do need to be taken seriously and do need to be given appropriate treatment."
Dr Maureen Marks, of the Institute of Psychiatry, told BBC News Online that post-natal depression was a complex condition, probably caused by many different factors.
"The more help we can give somebody with post-natal depression the better," she said.
"But lots of women with post-natal depression look after their children very well, in fact some over-compensate.
"So it would be wrong to think that a child born to a mother who has the condition is automatically at risk."