The notion that depression during pregnancy or after the birth of a baby is caused by hormones has been debunked by new research into the so-called baby blues.

A study published in medical journal The Lancet found that perinatal and postnatal depression was a continuation of mental health problems experienced well before a woman conceived.

Author Professor George Patton from the Murdoch Children's Research Institute said the long-term project tracked 2,000 young Victorians over 20 years.

"What we found is that 85 per cent of these women who experienced perinatal depression had a history of mental health problems, usually going back to their teen years," he said.

The findings challenge conventional thinking, Professor Patton said.

"For a long time we thought about perinatal depression as being something which is really quite unique," he said.

"That only occurred at this time in life, and it's something to do with the hormonal disturbances that go with pregnancy."

He said the effects of depression in pregnant women and new mothers can be profound.

"It has the potential to undermine the maternal infant bond which is so essential to the emotional development of the child."

It is hoped the research will help identify vulnerable women before they fall pregnant.

"What this really tells us as a study is that we need to think of perinatal depression very much in the life story," he said.

The authors said perinatal depression is a neglected global health priority affecting 10 to 15 per cent of women in wealthy countries and a higher proportion in developing countries.

The impacts for children can include cognitive, behavioural and emotional difficulties.

If you are suffering from perinatal depression call the National Perinatal Depression Helpline on 1300 726 306.