POSTNATAL depression, which brings untold anguish to thousands of homes every year, may be a relic from the days when humans lived in caves, scientists have discovered.
Researchers at Edinburgh University believe hormonal imbalances responsible for the devastating condition may have been responsible for increasing aggression in new mothers.
Dr Simone Meddle, a lecturer in veterinary science, claims that in prehistoric times the change in behaviour would help to prepare mothers for protecting their young from predators.
But as human life has become safer, these strong maternal instincts have become redundant. Instead, she says, the hormones may be building up and causing the overwhelming sense of despair and depression some mothers feel.
"There are huge changes in the blood and brain of a mother when she gives birth, to increase these maternal balances," said Meddle. "But while in our evolutionary past these may have been beneficial, now they can have a detrimental effect."
Meddle believes a chemical produced by the brain called vasopressin may be responsible for causing postnatal depression.
She has been given 250,000 from the Biotechnology and Biological Sciences Research Council for a three year study to unravel how the hormone works in rats.
She believes the large amounts of vasopressin build up to cause a malfunction in the brain circuitry when it is not burned off.
She said: "Female rats are normally quite docile and show no interest in other rats. But when they have offspring they become extremely defensive. Their aggression levels soar and they will attack intruders.
"Sometimes this aggression causes the mother rat to run on her own babies and harm them. Nobody really understands how it occurs and not many people are looking at it for some reason.
"By better understanding what is going on then we will hopefully develop ways of preventing postnatal depression all together."
Sometimes called the ‘Smiling Disease’, postnatal depression can leave mothers feeling hopeless after the birth of a child but they often find it difficult to share their feelings at a time when they are expected to be happy.
It can occur two to three months after birth and can last for years and even drive mothers to commit suicide or harm their baby. It is the major cause of death for mothers within a year of childbirth.
More than 11,000 women a year suffer from post-natal depression in Scotland but recent government figures suggest that the number could be even higher with up to one in five mothers suffering from the condition.
Debbie Persighetti, 41, a mother of two from Edinburgh, still struggles to talk about her postnatal depression due to guilt about how she felt.
She suffered from the condition after son Paolo was born five years ago and again following the birth of her daughter Eva 16 months ago.
But with the support of her husband John, she went to see her GP to get help. "It hit me like a bolt from the blue," she said. "I felt really tired and that I was struggling - like there was a black cloud I couldn’t get out from under.
"I didn’t want to get up in the morning as I didn’t know how I was going to get through the next 12 hours.
"Knowing that it is an imbalance in hormones over which I had no control and I was not going mad, was somehow comforting.
"I still have the odd bad day and my little boy has to ask why I am so sad. I know I have passed a corner and last week he said I had a happy day."
Last week a new campaign to raise awareness about the condition was launched. Bluebell Day, organised by the Church of Scotland’s Postnatal Depression Project, aims to increase the profile of the condition and raise funding for research.
Viv Dickenson, director of the project, said: "Too many women try to suffer in silence as they don’t want to make a fuss.
"They feel guilt at having negative thoughts at a time when they should be happy. They worry that others will think they are bad mothers.
"It is a terrible burden to carry around and can have a huge impact on family life for the children and the parents.
"Clearly what is behind this is very complex and that is why we need more research to find out what is going on."
The first Bluebell Day will take place on June 6 and will be accompanied by an advertising campaign.
Karen Westwood, who played a mother suffering from postnatal depression in the television series 2,000 Acres of Skye, is one of the supporters of the project.
Edinburgh-born TV star Gail Porter last week described how she broke down with the illness after giving birth to her daughter Honey.
Speaking on GMTV, the former Top of the Pops presenter revealed how she was prescribed the antidepressant Prozac after being unable to stop crying and constantly worrying she was a bad mum.
She said she had been bowled over from the support she has received and now knows a lot more about postnatal depression. "Lots of people suffer from it and there is nothing to be embarrassed about," she added.