The most severe form of postnatal depression, which affects one in 500 new mothers and has been linked to suicide and infanticide, could be genetic, according to new research.
It is also claimed, in a separate piece of research, that thousands more women could suffer postnatal depression than currently thought, with up to 17,250 late-onset cases a year in the UK going undetected.
It was believed that the mood disorders affecting up to 75 per cent of new mothers were caused by the women's circumstances, personality and hormonal changes.
But according to a study by Cardiff University, Birmingham University and Trinity College, Dublin, funded by medical charity the Wellcome Trust, the most severe form of postnatal depression - postpartum psychosis - has a genetic cause. The study is now working to isolate the gene, which will enable doctors to identify and treat high-risk women before they fall ill.
New mothers can suffer from a spectrum of mood disorders. But while most women suffer 'baby blues' - a short period of tearfulness and tiredness after childbirth - postnatal depression is a more severe, long-lasting condition which affects 10 to 15 per cent of women and can prevent mothers bonding with their babies and cause suicidal thoughts. If left untreated, it can affect the short or long-term development of the baby.
The impact on mothers can also be devastating. Women are 23 per cent more likely to be admitted to a psychiatric unit in the 18 months after giving birth than at any other time in their lives. Suicide is a leading cause of maternal death in the UK, with most attempts being made by those suffering an abrupt onset of postnatal depression.
Sufferers include Gwyneth Paltrow, Katie Price - aka Jordan - and Elle Macpherson, who has spoken of the extreme depression she experienced after the birth of her second child, Aurelius Cy, in 2003. Fern Britton has admitted feeling suicidal after the birth of her daughter Gracie. Brooke Shields had visions of her newborn baby Rowan being thrown against a wall.
The most serious form of maternal depression affects around one in 500 new mothers. Although rare, the condition has been associated with suicide and infanticide.
'Postpartum psychosis is classed as among the most severe episodes of illness seen in clinical practice,' said Dr Ian Jones, head of psychological medicine at Cardiff, who led the study into the DNA of families in which at least one woman had suffered postpartum psychosis. 'The consequences for the mother, infant and family are so serious that such episodes require close attention, often including hospitalisation.'
In a paper published in the November issue of the American Journal of Psychiatry, out this week, Jones will present his evidence, which has been peer reviewed, that women with a particular recessive gene are susceptible to postpartum psychosis.
'We have identified chromosomal regions that are likely to harbour genes that predispose individuals to bipolar affective puerperal psychosis,' he said. 'We have also been examining individual genes that code pregnancy and mood-related hormones or receptors, such as oestrogen, serotonin and oxytocin, and genes that have been suggested to be involved in other psychiatric conditions.' Jones's team will now carry out the first systematic genome scan to localise the genes that influence a woman's susceptibility to bipolar affective puerperal psychosis. 'It is hoped that identifying these factors will lead to improvements in the management of women who will become ill at this time and will increase understanding of affective disorders in general,' he said.
In a separate study to be published in Bipolar Disorders journal, Jessica Heron, a research fellow in Birmingham University's mental health research team, claims that a quarter of women who experience mild euphoria after childbirth go on to develop postnatal depression within two months.
Heron followed 500 women from their 12-week antenatal scan to eight weeks after they gave birth.
Her findings are backed by the responses of more than 1,000 mothers asked about postnatal depression by the website Mumsnet on behalf of The Observer. Of those who responded, 48 per cent said that they were not diagnosed for up to a year. A further 13 per cent suffered for between a year and 18 months before their condition was identified.
Almost 40 per cent of women said that they received no treatment, while of those who did receive help, 42 per cent said it was unsatisfactory.